Unit 6 — End of Topic Tests (reviewer copy: all keys and model answers visible)

[
 {
  "value": "SrO + N₂O₅ → SrNO₃",
  "misconception": "you gave strontium nitrate only one nitrate — Sr²⁺ needs two NO₃⁻ ions, written with parentheses as Sr(NO₃)₂",
  "feedback": "This answer is wrong — you gave strontium nitrate only one nitrate — Sr²⁺ needs two NO₃⁻ ions, written with parentheses as Sr(NO₃)₂"
 },
 {
  "value": "SrO + N₂O₅ → Sr(NO₂)₂",
  "misconception": "you wrote nitrite instead of nitrate — nitrate is the polyatomic ion NO₃⁻; NO₂⁻ is nitrite",
  "feedback": "This answer is wrong — you wrote nitrite instead of nitrate — nitrate is the polyatomic ion NO₃⁻; NO₂⁻ is nitrite"
 }
]

{
 "keyExplanation": "strontium oxide: Sr²⁺ and O²⁻ cancel one-to-one, giving SrO. Dinitrogen pentoxide is a molecular compound — its prefixes give the formula directly: N₂O₅. Strontium nitrate: Sr²⁺ needs two NO₃⁻ ions, written with parentheses as Sr(NO₃)₂. The translation is SrO + N₂O₅ → Sr(NO₃)₂.",
 "anticipatedWrong": [
  {
   "value": "SrO + N₂O₅ → SrNO₃",
   "misconception": "you gave strontium nitrate only one nitrate — Sr²⁺ needs two NO₃⁻ ions, written with parentheses as Sr(NO₃)₂",
   "feedback": "This answer is wrong — you gave strontium nitrate only one nitrate — Sr²⁺ needs two NO₃⁻ ions, written with parentheses as Sr(NO₃)₂"
  },
  {
   "value": "SrO + N₂O₅ → Sr(NO₂)₂",
   "misconception": "you wrote nitrite instead of nitrate — nitrate is the polyatomic ion NO₃⁻; NO₂⁻ is nitrite",
   "feedback": "This answer is wrong — you wrote nitrite instead of nitrate — nitrate is the polyatomic ion NO₃⁻; NO₂⁻ is nitrite"
  }
 ]
}

[
 {
  "value": 8,
  "misconception": "you ignored the coefficient — one formula unit holds 8 oxygens, and the 4 in front quadruples it",
  "feedback": "This answer is wrong — you ignored the coefficient — one formula unit holds 8 oxygens, and the 4 in front quadruples it"
 },
 {
  "value": 16,
  "misconception": "you forgot the outer subscript 2 — each formula unit holds two PO₄ groups, 8 oxygens, before the coefficient multiplies",
  "feedback": "This answer is wrong — you forgot the outer subscript 2 — each formula unit holds two PO₄ groups, 8 oxygens, before the coefficient multiplies"
 },
 {
  "value": 24,
  "misconception": "you multiplied by the calcium subscript instead of the coefficient — the 3 counts calcium atoms only",
  "feedback": "This answer is wrong — you multiplied by the calcium subscript instead of the coefficient — the 3 counts calcium atoms only"
 }
]

{
 "keyExplanation": "Count inside one formula unit first: each PO₄ holds 4 oxygens, and the outer subscript 2 makes 2 × 4 = 8. Multiply by the coefficient: 4 × 8 = 32 oxygen atoms.",
 "anticipatedWrong": [
  {
   "value": 8,
   "misconception": "you ignored the coefficient — one formula unit holds 8 oxygens, and the 4 in front quadruples it",
   "feedback": "This answer is wrong — you ignored the coefficient — one formula unit holds 8 oxygens, and the 4 in front quadruples it"
  },
  {
   "value": 16,
   "misconception": "you forgot the outer subscript 2 — each formula unit holds two PO₄ groups, 8 oxygens, before the coefficient multiplies",
   "feedback": "This answer is wrong — you forgot the outer subscript 2 — each formula unit holds two PO₄ groups, 8 oxygens, before the coefficient multiplies"
  },
  {
   "value": 24,
   "misconception": "you multiplied by the calcium subscript instead of the coefficient — the 3 counts calcium atoms only",
   "feedback": "This answer is wrong — you multiplied by the calcium subscript instead of the coefficient — the 3 counts calcium atoms only"
  }
 ]
}

[
 "Write down the values in the question:",
 "mass of Br₂ = 8.0 g",
 "mass of LiI = 13.4 g",
 "mass of I₂ = 12.7 g",
 "Write down the equation:",
 "total mass of reactants = total mass of products",
 "In this problem's substances:",
 "mass of Br₂ + mass of LiI = mass of LiBr + mass of I₂",
 "Make the mass of LiBr the subject:",
 "mass of LiBr = (mass of Br₂ + mass of LiI) − mass of I₂",
 "Substitute in the values, and calculate:",
 "mass of LiBr = (8.0 + 13.4) − 12.7",
 "mass of LiBr = 21.4 − 12.7",
 "mass of LiBr = 8.7 g"
]

[
 {
  "value": 21.4,
  "misconception": "you found the reactant total and stopped — subtract the I₂ from that total to leave the LiBr",
  "feedback": "This answer is wrong — you found the reactant total and stopped — subtract the I₂ from that total to leave the LiBr"
 },
 {
  "value": 0.7,
  "misconception": "you subtracted the I₂ from the LiI alone — total BOTH reactants first",
  "feedback": "This answer is wrong — you subtracted the I₂ from the LiI alone — total BOTH reactants first"
 },
 {
  "value": 4.7,
  "misconception": "you subtracted the Br₂ from the I₂ — the Br₂ is a reactant and belongs in the reactant total",
  "feedback": "This answer is wrong — you subtracted the Br₂ from the I₂ — the Br₂ is a reactant and belongs in the reactant total"
 }
]

{
 "keyExplanation": "Write down the values in the question:\nmass of Br₂ = 8.0 g\nmass of LiI = 13.4 g\nmass of I₂ = 12.7 g\nWrite down the equation:\ntotal mass of reactants = total mass of products\nIn this problem's substances:\nmass of Br₂ + mass of LiI = mass of LiBr + mass of I₂\nMake the mass of LiBr the subject:\nmass of LiBr = (mass of Br₂ + mass of LiI) − mass of I₂\nSubstitute in the values, and calculate:\nmass of LiBr = (8.0 + 13.4) − 12.7\nmass of LiBr = 21.4 − 12.7\nmass of LiBr = 8.7 g",
 "anticipatedWrong": [
  {
   "value": 21.4,
   "misconception": "you found the reactant total and stopped — subtract the I₂ from that total to leave the LiBr",
   "feedback": "This answer is wrong — you found the reactant total and stopped — subtract the I₂ from that total to leave the LiBr"
  },
  {
   "value": 0.7,
   "misconception": "you subtracted the I₂ from the LiI alone — total BOTH reactants first",
   "feedback": "This answer is wrong — you subtracted the I₂ from the LiI alone — total BOTH reactants first"
  },
  {
   "value": 4.7,
   "misconception": "you subtracted the Br₂ from the I₂ — the Br₂ is a reactant and belongs in the reactant total",
   "feedback": "This answer is wrong — you subtracted the Br₂ from the I₂ — the Br₂ is a reactant and belongs in the reactant total"
  }
 ]
}

[
 {
  "value": "Ca₃ + N₂ → Ca₃N₂",
  "misconception": "you wrote calcium's count as a subscript on the element — three separate calcium atoms are written 3Ca, with a coefficient",
  "feedback": "This answer is wrong — you wrote calcium's count as a subscript on the element — three separate calcium atoms are written 3Ca, with a coefficient"
 },
 {
  "value": "Ca + N₂ → CaN₂",
  "misconception": "you changed the product's subscript — Ca₃N₂ is fixed by the compound itself; the fix is a coefficient on Ca",
  "feedback": "This answer is wrong — you changed the product's subscript — Ca₃N₂ is fixed by the compound itself; the fix is a coefficient on Ca"
 },
 {
  "value": "6Ca + 2N₂ → 2Ca₃N₂",
  "misconception": "you doubled a set that already worked — use the smallest whole-number coefficients: 3, 1, 1",
  "feedback": "This answer is wrong — you doubled a set that already worked — use the smallest whole-number coefficients: 3, 1, 1"
 }
]

{
 "keyExplanation": "Count: calcium 1 and 3, nitrogen 2 and 2. Nitrogen already matches. Adjust calcium: place a 3 in front of Ca. Recount: calcium 3 and 3, nitrogen 2 and 2 — 3Ca + N₂ → Ca₃N₂.",
 "anticipatedWrong": [
  {
   "value": "Ca₃ + N₂ → Ca₃N₂",
   "misconception": "you wrote calcium's count as a subscript on the element — three separate calcium atoms are written 3Ca, with a coefficient",
   "feedback": "This answer is wrong — you wrote calcium's count as a subscript on the element — three separate calcium atoms are written 3Ca, with a coefficient"
  },
  {
   "value": "Ca + N₂ → CaN₂",
   "misconception": "you changed the product's subscript — Ca₃N₂ is fixed by the compound itself; the fix is a coefficient on Ca",
   "feedback": "This answer is wrong — you changed the product's subscript — Ca₃N₂ is fixed by the compound itself; the fix is a coefficient on Ca"
  },
  {
   "value": "6Ca + 2N₂ → 2Ca₃N₂",
   "misconception": "you doubled a set that already worked — use the smallest whole-number coefficients: 3, 1, 1",
   "feedback": "This answer is wrong — you doubled a set that already worked — use the smallest whole-number coefficients: 3, 1, 1"
  }
 ]
}

[
 {
  "value": "3AgNO₃ + Na₃PO₄ → Ag₃PO₄ + NaNO₃",
  "misconception": "you balanced silver but never recounted nitrate — 3AgNO₃ carries 3 nitrate units, so the right side needs 3NaNO₃",
  "feedback": "This answer is wrong — you balanced silver but never recounted nitrate — 3AgNO₃ carries 3 nitrate units, so the right side needs 3NaNO₃"
 },
 {
  "value": "Ag(NO₃)₃ + Na₃PO₄ → Ag₃PO₄ + 3NaNO₃",
  "misconception": "you changed a formula — Ag(NO₃)₃ is not silver nitrate; silver's fix is the coefficient 3, never a new subscript",
  "feedback": "This answer is wrong — you changed a formula — Ag(NO₃)₃ is not silver nitrate; silver's fix is the coefficient 3, never a new subscript"
 },
 {
  "value": "AgNO₃ + Na₃PO₄ → Ag₃PO₄ + NaNO₃",
  "misconception": "you left everything at 1 — count the units: silver is 1 against 3, sodium 3 against 1; coefficients of 3 on AgNO₃ and NaNO₃ fix all four counts",
  "feedback": "This answer is wrong — you left everything at 1 — count the units: silver is 1 against 3, sodium 3 against 1; coefficients of 3 on AgNO₃ and NaNO₃ fix all four counts"
 }
]

{
 "keyExplanation": "Nitrate and phosphate both appear unchanged — count them as units. Count: silver 1 and 3, nitrate 1 and 1, sodium 3 and 1, phosphate 1 and 1. Place a 3 in front of AgNO₃ for silver; recount — nitrate is now 3 and 1, so place a 3 in front of NaNO₃; recount — sodium 3 and 3, everything matches: 3AgNO₃ + Na₃PO₄ → Ag₃PO₄ + 3NaNO₃.",
 "anticipatedWrong": [
  {
   "value": "3AgNO₃ + Na₃PO₄ → Ag₃PO₄ + NaNO₃",
   "misconception": "you balanced silver but never recounted nitrate — 3AgNO₃ carries 3 nitrate units, so the right side needs 3NaNO₃",
   "feedback": "This answer is wrong — you balanced silver but never recounted nitrate — 3AgNO₃ carries 3 nitrate units, so the right side needs 3NaNO₃"
  },
  {
   "value": "Ag(NO₃)₃ + Na₃PO₄ → Ag₃PO₄ + 3NaNO₃",
   "misconception": "you changed a formula — Ag(NO₃)₃ is not silver nitrate; silver's fix is the coefficient 3, never a new subscript",
   "feedback": "This answer is wrong — you changed a formula — Ag(NO₃)₃ is not silver nitrate; silver's fix is the coefficient 3, never a new subscript"
  },
  {
   "value": "AgNO₃ + Na₃PO₄ → Ag₃PO₄ + NaNO₃",
   "misconception": "you left everything at 1 — count the units: silver is 1 against 3, sodium 3 against 1; coefficients of 3 on AgNO₃ and NaNO₃ fix all four counts",
   "feedback": "This answer is wrong — you left everything at 1 — count the units: silver is 1 against 3, sodium 3 against 1; coefficients of 3 on AgNO₃ and NaNO₃ fix all four counts"
  }
 ]
}

[
 {
  "value": "C₄H₁₀ + 7O₂ → 4CO₂ + 5H₂O",
  "misconception": "you rounded the odd oxygen count instead of doubling — 4CO₂ + 5H₂O holds 13 oxygens, and 7O₂ would supply 14; double every coefficient instead",
  "feedback": "This answer is wrong — you rounded the odd oxygen count instead of doubling — 4CO₂ + 5H₂O holds 13 oxygens, and 7O₂ would supply 14; double every coefficient instead"
 },
 {
  "value": "C₄H₁₀ + 13O₂ → 4CO₂ + 5H₂O",
  "misconception": "you doubled only the O₂ term — the doubling move multiplies EVERY coefficient, then O₂ is set from the new right side",
  "feedback": "This answer is wrong — you doubled only the O₂ term — the doubling move multiplies EVERY coefficient, then O₂ is set from the new right side"
 },
 {
  "value": "2C₄H₁₀ + 13O₂ → 8CO₂ + 5H₂O",
  "misconception": "you halved the hydrogen twice — 2C₄H₁₀ carries 20 hydrogens, needing 10H₂O, not 5",
  "feedback": "This answer is wrong — you halved the hydrogen twice — 2C₄H₁₀ carries 20 hydrogens, needing 10H₂O, not 5"
 }
]

{
 "keyExplanation": "Carbon: 4CO₂. Hydrogen: half of 10 gives 5H₂O. The right side then holds 8 + 5 = 13 oxygen atoms — odd, so double the fuel, CO₂, and H₂O coefficients (O₂ is still unset): 2C₄H₁₀ + O₂ → 8CO₂ + 10H₂O. Oxygen last: 16 + 10 = 26, so 13O₂. Check: carbon 8 and 8, hydrogen 20 and 20, oxygen 26 and 26.",
 "anticipatedWrong": [
  {
   "value": "C₄H₁₀ + 7O₂ → 4CO₂ + 5H₂O",
   "misconception": "you rounded the odd oxygen count instead of doubling — 4CO₂ + 5H₂O holds 13 oxygens, and 7O₂ would supply 14; double every coefficient instead",
   "feedback": "This answer is wrong — you rounded the odd oxygen count instead of doubling — 4CO₂ + 5H₂O holds 13 oxygens, and 7O₂ would supply 14; double every coefficient instead"
  },
  {
   "value": "C₄H₁₀ + 13O₂ → 4CO₂ + 5H₂O",
   "misconception": "you doubled only the O₂ term — the doubling move multiplies EVERY coefficient, then O₂ is set from the new right side",
   "feedback": "This answer is wrong — you doubled only the O₂ term — the doubling move multiplies EVERY coefficient, then O₂ is set from the new right side"
  },
  {
   "value": "2C₄H₁₀ + 13O₂ → 8CO₂ + 5H₂O",
   "misconception": "you halved the hydrogen twice — 2C₄H₁₀ carries 20 hydrogens, needing 10H₂O, not 5",
   "feedback": "This answer is wrong — you halved the hydrogen twice — 2C₄H₁₀ carries 20 hydrogens, needing 10H₂O, not 5"
  }
 ]
}

[
 {
  "value": "BaCl₂ + K₂SO₄ → BaSO₄ + KCl",
  "misconception": "you translated correctly but skipped the balancing — potassium and chlorine both stand at 2 against 1, and one 2 before KCl fixes both",
  "feedback": "This answer is wrong — you translated correctly but skipped the balancing — potassium and chlorine both stand at 2 against 1, and one 2 before KCl fixes both"
 },
 {
  "value": "BaCl₂ + K₂SO₄ → BaSO₄ + KCl₂",
  "misconception": "you changed a formula — KCl₂ does not exist; potassium chloride is KCl, and the fix is the coefficient 2",
  "feedback": "This answer is wrong — you changed a formula — KCl₂ does not exist; potassium chloride is KCl, and the fix is the coefficient 2"
 },
 {
  "value": "BaCl₂ + K₂SO₄ → Ba(SO₄)₂ + 2KCl",
  "misconception": "you gave barium sulfate a doubled formula — from Ba²⁺ and SO₄²⁻ the charges cancel one to one, so the product is BaSO₄",
  "feedback": "This answer is wrong — you gave barium sulfate a doubled formula — from Ba²⁺ and SO₄²⁻ the charges cancel one to one, so the product is BaSO₄"
 }
]

{
 "keyExplanation": "Translate: barium chloride is BaCl₂, potassium sulfate — two K⁺ with SO₄²⁻ — is K₂SO₄, barium sulfate is BaSO₄, potassium chloride is KCl. Skeleton: BaCl₂ + K₂SO₄ → BaSO₄ + KCl. Sulfate appears unchanged — count it as a unit: barium 1 and 1, sulfate 1 and 1, potassium 2 and 1, chlorine 2 and 1. Place a 2 before KCl. Check: BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl — every count matches.",
 "anticipatedWrong": [
  {
   "value": "BaCl₂ + K₂SO₄ → BaSO₄ + KCl",
   "misconception": "you translated correctly but skipped the balancing — potassium and chlorine both stand at 2 against 1, and one 2 before KCl fixes both",
   "feedback": "This answer is wrong — you translated correctly but skipped the balancing — potassium and chlorine both stand at 2 against 1, and one 2 before KCl fixes both"
  },
  {
   "value": "BaCl₂ + K₂SO₄ → BaSO₄ + KCl₂",
   "misconception": "you changed a formula — KCl₂ does not exist; potassium chloride is KCl, and the fix is the coefficient 2",
   "feedback": "This answer is wrong — you changed a formula — KCl₂ does not exist; potassium chloride is KCl, and the fix is the coefficient 2"
  },
  {
   "value": "BaCl₂ + K₂SO₄ → Ba(SO₄)₂ + 2KCl",
   "misconception": "you gave barium sulfate a doubled formula — from Ba²⁺ and SO₄²⁻ the charges cancel one to one, so the product is BaSO₄",
   "feedback": "This answer is wrong — you gave barium sulfate a doubled formula — from Ba²⁺ and SO₄²⁻ the charges cancel one to one, so the product is BaSO₄"
  }
 ]
}

[
 {
  "value": "Ag(NO₃)₂ + NaCl → AgCl + NaNO₃",
  "misconception": "you doubled the nitrate — Ag⁺ and NO₃⁻ cancel one-to-one, so silver nitrate is AgNO₃",
  "feedback": "This answer is wrong — you doubled the nitrate — Ag⁺ and NO₃⁻ cancel one-to-one, so silver nitrate is AgNO₃"
 },
 {
  "value": "AgN + NaCl → AgCl + NaN",
  "misconception": "you wrote nitride instead of nitrate — nitrate is the polyatomic ion NO₃⁻",
  "feedback": "This answer is wrong — you wrote nitride instead of nitrate — nitrate is the polyatomic ion NO₃⁻"
 }
]

{
 "keyExplanation": "silver nitrate: Ag⁺ and NO₃⁻ give AgNO₃. Sodium chloride is NaCl. Silver chloride: Ag⁺ and Cl⁻ give AgCl. Sodium nitrate: Na⁺ and NO₃⁻ give NaNO₃. The translation is AgNO₃ + NaCl → AgCl + NaNO₃.",
 "anticipatedWrong": [
  {
   "value": "Ag(NO₃)₂ + NaCl → AgCl + NaNO₃",
   "misconception": "you doubled the nitrate — Ag⁺ and NO₃⁻ cancel one-to-one, so silver nitrate is AgNO₃",
   "feedback": "This answer is wrong — you doubled the nitrate — Ag⁺ and NO₃⁻ cancel one-to-one, so silver nitrate is AgNO₃"
  },
  {
   "value": "AgN + NaCl → AgCl + NaN",
   "misconception": "you wrote nitride instead of nitrate — nitrate is the polyatomic ion NO₃⁻",
   "feedback": "This answer is wrong — you wrote nitride instead of nitrate — nitrate is the polyatomic ion NO₃⁻"
  }
 ]
}

[
 {
  "value": 3,
  "misconception": "you counted one nitrogen per formula unit — NH₄NO₃ holds nitrogen in two places, one in NH₄ and one in NO₃",
  "feedback": "This answer is wrong — you counted one nitrogen per formula unit — NH₄NO₃ holds nitrogen in two places, one in NH₄ and one in NO₃"
 },
 {
  "value": 2,
  "misconception": "you ignored the coefficient — one formula unit holds 2 nitrogens, and the 3 in front triples it",
  "feedback": "This answer is wrong — you ignored the coefficient — one formula unit holds 2 nitrogens, and the 3 in front triples it"
 },
 {
  "value": 12,
  "misconception": "you multiplied the coefficient by the hydrogen subscript — the 4 counts hydrogens, not nitrogens",
  "feedback": "This answer is wrong — you multiplied the coefficient by the hydrogen subscript — the 4 counts hydrogens, not nitrogens"
 }
]

{
 "keyExplanation": "One NH₄NO₃ formula unit holds 2 nitrogen atoms — one in the NH₄ and one in the NO₃. Multiply by the coefficient: 3 × 2 = 6 nitrogen atoms.",
 "anticipatedWrong": [
  {
   "value": 3,
   "misconception": "you counted one nitrogen per formula unit — NH₄NO₃ holds nitrogen in two places, one in NH₄ and one in NO₃",
   "feedback": "This answer is wrong — you counted one nitrogen per formula unit — NH₄NO₃ holds nitrogen in two places, one in NH₄ and one in NO₃"
  },
  {
   "value": 2,
   "misconception": "you ignored the coefficient — one formula unit holds 2 nitrogens, and the 3 in front triples it",
   "feedback": "This answer is wrong — you ignored the coefficient — one formula unit holds 2 nitrogens, and the 3 in front triples it"
  },
  {
   "value": 12,
   "misconception": "you multiplied the coefficient by the hydrogen subscript — the 4 counts hydrogens, not nitrogens",
   "feedback": "This answer is wrong — you multiplied the coefficient by the hydrogen subscript — the 4 counts hydrogens, not nitrogens"
  }
 ]
}

[
 "Write down the values in the question:",
 "mass of H₂ = 1.0 g",
 "mass of O₂ = 8.0 g",
 "Write down the equation:",
 "total mass of reactants = total mass of products",
 "In this problem's substances:",
 "mass of H₂O = mass of H₂ + mass of O₂",
 "The mass of H₂O is already the subject, so substitute straight in, and calculate:",
 "mass of H₂O = 1.0 + 8.0",
 "mass of H₂O = 9.0 g"
]

[
 {
  "value": 7.0,
  "misconception": "you subtracted the product masses — the H₂O supplied BOTH products, so add them",
  "feedback": "This answer is wrong — you subtracted the product masses — the H₂O supplied BOTH products, so add them"
 },
 {
  "value": 8.0,
  "misconception": "you counted only the O₂ — the H₂'s 1.0 g also came from the H₂O",
  "feedback": "This answer is wrong — you counted only the O₂ — the H₂'s 1.0 g also came from the H₂O"
 },
 {
  "value": 1.0,
  "misconception": "you counted only the H₂ — the O₂'s 8.0 g also came from the H₂O",
  "feedback": "This answer is wrong — you counted only the H₂ — the O₂'s 8.0 g also came from the H₂O"
 }
]

{
 "keyExplanation": "Write down the values in the question:\nmass of H₂ = 1.0 g\nmass of O₂ = 8.0 g\nWrite down the equation:\ntotal mass of reactants = total mass of products\nIn this problem's substances:\nmass of H₂O = mass of H₂ + mass of O₂\nThe mass of H₂O is already the subject, so substitute straight in, and calculate:\nmass of H₂O = 1.0 + 8.0\nmass of H₂O = 9.0 g",
 "anticipatedWrong": [
  {
   "value": 7.0,
   "misconception": "you subtracted the product masses — the H₂O supplied BOTH products, so add them",
   "feedback": "This answer is wrong — you subtracted the product masses — the H₂O supplied BOTH products, so add them"
  },
  {
   "value": 8.0,
   "misconception": "you counted only the O₂ — the H₂'s 1.0 g also came from the H₂O",
   "feedback": "This answer is wrong — you counted only the O₂ — the H₂'s 1.0 g also came from the H₂O"
  },
  {
   "value": 1.0,
   "misconception": "you counted only the H₂ — the O₂'s 8.0 g also came from the H₂O",
   "feedback": "This answer is wrong — you counted only the H₂ — the O₂'s 8.0 g also came from the H₂O"
  }
 ]
}

[
 {
  "value": "Zn + HCl₂ → ZnCl₂ + H₂",
  "misconception": "you changed a subscript — HCl₂ is not hydrochloric acid; the fix is a coefficient of 2 in front of HCl",
  "feedback": "This answer is wrong — you changed a subscript — HCl₂ is not hydrochloric acid; the fix is a coefficient of 2 in front of HCl"
 },
 {
  "value": "Zn + 2HCl → ZnCl₂ + 2H₂",
  "misconception": "you balanced chlorine but broke hydrogen — check the recount: 2HCl gives hydrogen 2 and 2 as well, so no coefficient belongs on H₂",
  "feedback": "This answer is wrong — you balanced chlorine but broke hydrogen — check the recount: 2HCl gives hydrogen 2 and 2 as well, so no coefficient belongs on H₂"
 },
 {
  "value": "2Zn + 4HCl → 2ZnCl₂ + 2H₂",
  "misconception": "you doubled a set that already worked — use the smallest whole-number coefficients: 1, 2, 1, 1",
  "feedback": "This answer is wrong — you doubled a set that already worked — use the smallest whole-number coefficients: 1, 2, 1, 1"
 }
]

{
 "keyExplanation": "Count: zinc 1 and 1, hydrogen 1 and 2, chlorine 1 and 2. Place a 2 in front of HCl to fix both hydrogen and chlorine at once. Recount: zinc 1 and 1, hydrogen 2 and 2, chlorine 2 and 2 — Zn + 2HCl → ZnCl₂ + H₂.",
 "anticipatedWrong": [
  {
   "value": "Zn + HCl₂ → ZnCl₂ + H₂",
   "misconception": "you changed a subscript — HCl₂ is not hydrochloric acid; the fix is a coefficient of 2 in front of HCl",
   "feedback": "This answer is wrong — you changed a subscript — HCl₂ is not hydrochloric acid; the fix is a coefficient of 2 in front of HCl"
  },
  {
   "value": "Zn + 2HCl → ZnCl₂ + 2H₂",
   "misconception": "you balanced chlorine but broke hydrogen — check the recount: 2HCl gives hydrogen 2 and 2 as well, so no coefficient belongs on H₂",
   "feedback": "This answer is wrong — you balanced chlorine but broke hydrogen — check the recount: 2HCl gives hydrogen 2 and 2 as well, so no coefficient belongs on H₂"
  },
  {
   "value": "2Zn + 4HCl → 2ZnCl₂ + 2H₂",
   "misconception": "you doubled a set that already worked — use the smallest whole-number coefficients: 1, 2, 1, 1",
   "feedback": "This answer is wrong — you doubled a set that already worked — use the smallest whole-number coefficients: 1, 2, 1, 1"
  }
 ]
}

[
 {
  "value": "Fe(NO₃)₃ + 3NaOH → Fe(OH)₃ + NaNO₃",
  "misconception": "you fixed hydroxide but never recounted nitrate — Fe(NO₃)₃ carries 3 nitrate units, so the right side needs 3NaNO₃",
  "feedback": "This answer is wrong — you fixed hydroxide but never recounted nitrate — Fe(NO₃)₃ carries 3 nitrate units, so the right side needs 3NaNO₃"
 },
 {
  "value": "Fe(NO₃)₃ + Na(OH)₃ → Fe(OH)₃ + 3NaNO₃",
  "misconception": "you changed a formula — Na(OH)₃ does not exist; hydroxide's fix is the coefficient 3 in front of NaOH",
  "feedback": "This answer is wrong — you changed a formula — Na(OH)₃ does not exist; hydroxide's fix is the coefficient 3 in front of NaOH"
 },
 {
  "value": "Fe(NO₃)₃ + NaOH → Fe(OH)₃ + NaNO₃",
  "misconception": "you left the counts at 1 — nitrate is 3 against 1 and hydroxide 1 against 3; coefficients of 3 on NaOH and NaNO₃ fix them",
  "feedback": "This answer is wrong — you left the counts at 1 — nitrate is 3 against 1 and hydroxide 1 against 3; coefficients of 3 on NaOH and NaNO₃ fix them"
 }
]

{
 "keyExplanation": "Nitrate and hydroxide both appear unchanged — count them as units. Count: iron 1 and 1, nitrate 3 and 1, sodium 1 and 1, hydroxide 1 and 3. Place a 3 in front of NaOH for hydroxide; recount — sodium is 3 and 1, so place a 3 in front of NaNO₃; recount — nitrate 3 and 3, sodium 3 and 3: Fe(NO₃)₃ + 3NaOH → Fe(OH)₃ + 3NaNO₃.",
 "anticipatedWrong": [
  {
   "value": "Fe(NO₃)₃ + 3NaOH → Fe(OH)₃ + NaNO₃",
   "misconception": "you fixed hydroxide but never recounted nitrate — Fe(NO₃)₃ carries 3 nitrate units, so the right side needs 3NaNO₃",
   "feedback": "This answer is wrong — you fixed hydroxide but never recounted nitrate — Fe(NO₃)₃ carries 3 nitrate units, so the right side needs 3NaNO₃"
  },
  {
   "value": "Fe(NO₃)₃ + Na(OH)₃ → Fe(OH)₃ + 3NaNO₃",
   "misconception": "you changed a formula — Na(OH)₃ does not exist; hydroxide's fix is the coefficient 3 in front of NaOH",
   "feedback": "This answer is wrong — you changed a formula — Na(OH)₃ does not exist; hydroxide's fix is the coefficient 3 in front of NaOH"
  },
  {
   "value": "Fe(NO₃)₃ + NaOH → Fe(OH)₃ + NaNO₃",
   "misconception": "you left the counts at 1 — nitrate is 3 against 1 and hydroxide 1 against 3; coefficients of 3 on NaOH and NaNO₃ fix them",
   "feedback": "This answer is wrong — you left the counts at 1 — nitrate is 3 against 1 and hydroxide 1 against 3; coefficients of 3 on NaOH and NaNO₃ fix them"
  }
 ]
}

[
 {
  "value": "C₆H₁₀ + 8O₂ → 6CO₂ + 5H₂O",
  "misconception": "you left the odd oxygen count standing — 6CO₂ + 5H₂O holds 17 oxygens, and no whole O₂ coefficient supplies 17; double every coefficient first",
  "feedback": "This answer is wrong — you left the odd oxygen count standing — 6CO₂ + 5H₂O holds 17 oxygens, and no whole O₂ coefficient supplies 17; double every coefficient first"
 },
 {
  "value": "C₆H₁₀ + 8.5O₂ → 6CO₂ + 5H₂O",
  "misconception": "you wrote a fractional coefficient — 8.5 O₂ balances the count, but coefficients must be whole numbers; double everything instead",
  "feedback": "This answer is wrong — you wrote a fractional coefficient — 8.5 O₂ balances the count, but coefficients must be whole numbers; double everything instead"
 },
 {
  "value": "C₆H₁₀ + 17O₂ → 6CO₂ + 5H₂O",
  "misconception": "you doubled only the O₂ term — the doubling move multiplies EVERY coefficient, then O₂ is set from the new right side",
  "feedback": "This answer is wrong — you doubled only the O₂ term — the doubling move multiplies EVERY coefficient, then O₂ is set from the new right side"
 }
]

{
 "keyExplanation": "Carbon: 6CO₂. Hydrogen: half of 10 gives 5H₂O. The right side then holds 12 + 5 = 17 oxygen atoms — odd, so double the fuel, CO₂, and H₂O coefficients (O₂ is still unset): 2C₆H₁₀ + O₂ → 12CO₂ + 10H₂O. Oxygen last: 24 + 10 = 34, so 17O₂. Check: carbon 12 and 12, hydrogen 20 and 20, oxygen 34 and 34.",
 "anticipatedWrong": [
  {
   "value": "C₆H₁₀ + 8O₂ → 6CO₂ + 5H₂O",
   "misconception": "you left the odd oxygen count standing — 6CO₂ + 5H₂O holds 17 oxygens, and no whole O₂ coefficient supplies 17; double every coefficient first",
   "feedback": "This answer is wrong — you left the odd oxygen count standing — 6CO₂ + 5H₂O holds 17 oxygens, and no whole O₂ coefficient supplies 17; double every coefficient first"
  },
  {
   "value": "C₆H₁₀ + 8.5O₂ → 6CO₂ + 5H₂O",
   "misconception": "you wrote a fractional coefficient — 8.5 O₂ balances the count, but coefficients must be whole numbers; double everything instead",
   "feedback": "This answer is wrong — you wrote a fractional coefficient — 8.5 O₂ balances the count, but coefficients must be whole numbers; double everything instead"
  },
  {
   "value": "C₆H₁₀ + 17O₂ → 6CO₂ + 5H₂O",
   "misconception": "you doubled only the O₂ term — the doubling move multiplies EVERY coefficient, then O₂ is set from the new right side",
   "feedback": "This answer is wrong — you doubled only the O₂ term — the doubling move multiplies EVERY coefficient, then O₂ is set from the new right side"
  }
 ]
}

[
 {
  "value": "C₉H₂₀ + 9O₂ → 9CO₂ + 10H₂O",
  "misconception": "you set O₂ from the CO₂ count alone — the water's oxygens count too: 18 + 10 = 28, so 14O₂",
  "feedback": "This answer is wrong — you set O₂ from the CO₂ count alone — the water's oxygens count too: 18 + 10 = 28, so 14O₂"
 },
 {
  "value": "C₉H₂₀ + 19O₂ → 9CO₂ + 20H₂O",
  "misconception": "you copied the hydrogen subscript straight onto H₂O — each water already holds 2 hydrogens, so 20 hydrogens need 10H₂O",
  "feedback": "This answer is wrong — you copied the hydrogen subscript straight onto H₂O — each water already holds 2 hydrogens, so 20 hydrogens need 10H₂O"
 },
 {
  "value": "2C₉H₂₀ + 28O₂ → 18CO₂ + 20H₂O",
  "misconception": "you doubled a set that already worked — the oxygen count 28 is even, so no doubling is needed",
  "feedback": "This answer is wrong — you doubled a set that already worked — the oxygen count 28 is even, so no doubling is needed"
 }
]

{
 "keyExplanation": "Translate: nonane is C₉H₂₀, oxygen gas is O₂, carbon dioxide is CO₂, and water is H₂O. Carbon first: 9CO₂. Hydrogen next: half of 20 gives 10H₂O. The right side then holds 18 + 10 = 28 oxygen atoms — even, so no doubling is needed: 14O₂. Check: carbon 9 and 9, hydrogen 20 and 20, oxygen 28 and 28.",
 "anticipatedWrong": [
  {
   "value": "C₉H₂₀ + 9O₂ → 9CO₂ + 10H₂O",
   "misconception": "you set O₂ from the CO₂ count alone — the water's oxygens count too: 18 + 10 = 28, so 14O₂",
   "feedback": "This answer is wrong — you set O₂ from the CO₂ count alone — the water's oxygens count too: 18 + 10 = 28, so 14O₂"
  },
  {
   "value": "C₉H₂₀ + 19O₂ → 9CO₂ + 20H₂O",
   "misconception": "you copied the hydrogen subscript straight onto H₂O — each water already holds 2 hydrogens, so 20 hydrogens need 10H₂O",
   "feedback": "This answer is wrong — you copied the hydrogen subscript straight onto H₂O — each water already holds 2 hydrogens, so 20 hydrogens need 10H₂O"
  },
  {
   "value": "2C₉H₂₀ + 28O₂ → 18CO₂ + 20H₂O",
   "misconception": "you doubled a set that already worked — the oxygen count 28 is even, so no doubling is needed",
   "feedback": "This answer is wrong — you doubled a set that already worked — the oxygen count 28 is even, so no doubling is needed"
  }
 ]
}

[
 {
  "value": "BaO + N₂O₅ → BaNO₃",
  "misconception": "you dropped the parentheses and the subscript — Ba²⁺ needs two NO₃⁻ ions, written Ba(NO₃)₂",
  "feedback": "This answer is wrong — you dropped the parentheses and the subscript — Ba²⁺ needs two NO₃⁻ ions, written Ba(NO₃)₂"
 },
 {
  "value": "BaO + N₂O₅ → Ba₂NO₃",
  "misconception": "you put the 2 on the barium instead of on the nitrate — the 2+ charge on Ba²⁺ tells you it takes two NO₃⁻ ions: Ba(NO₃)₂, not two bariums",
  "feedback": "This answer is wrong — you put the 2 on the barium instead of on the nitrate — the 2+ charge on Ba²⁺ tells you it takes two NO₃⁻ ions: Ba(NO₃)₂, not two bariums"
 }
]

{
 "keyExplanation": "barium oxide: Ba²⁺ and O²⁻ cancel one-to-one, giving BaO. Dinitrogen pentoxide is a molecular compound — its prefixes give the formula directly: N₂O₅. Barium nitrate: Ba²⁺ needs two NO₃⁻ ions, written with parentheses as Ba(NO₃)₂. The translation is BaO + N₂O₅ → Ba(NO₃)₂.",
 "anticipatedWrong": [
  {
   "value": "BaO + N₂O₅ → BaNO₃",
   "misconception": "you dropped the parentheses and the subscript — Ba²⁺ needs two NO₃⁻ ions, written Ba(NO₃)₂",
   "feedback": "This answer is wrong — you dropped the parentheses and the subscript — Ba²⁺ needs two NO₃⁻ ions, written Ba(NO₃)₂"
  },
  {
   "value": "BaO + N₂O₅ → Ba₂NO₃",
   "misconception": "you put the 2 on the barium instead of on the nitrate — the 2+ charge on Ba²⁺ tells you it takes two NO₃⁻ ions: Ba(NO₃)₂, not two bariums",
   "feedback": "This answer is wrong — you put the 2 on the barium instead of on the nitrate — the 2+ charge on Ba²⁺ tells you it takes two NO₃⁻ ions: Ba(NO₃)₂, not two bariums"
  }
 ]
}

[
 {
  "value": 7,
  "misconception": "you ignored the coefficient — one H₂SO₄ holds 7 atoms, and the 2 in front doubles it",
  "feedback": "This answer is wrong — you ignored the coefficient — one H₂SO₄ holds 7 atoms, and the 2 in front doubles it"
 },
 {
  "value": 12,
  "misconception": "you skipped the sulfur — it carries no subscript, so it counts as 1 atom per formula unit",
  "feedback": "This answer is wrong — you skipped the sulfur — it carries no subscript, so it counts as 1 atom per formula unit"
 },
 {
  "value": 9,
  "misconception": "you added the coefficient instead of multiplying by it",
  "feedback": "This answer is wrong — you added the coefficient instead of multiplying by it"
 }
]

{
 "keyExplanation": "Count one formula unit: 2 hydrogens + 1 sulfur + 4 oxygens = 7 atoms. Multiply by the coefficient: 2 × 7 = 14 atoms in total.",
 "anticipatedWrong": [
  {
   "value": 7,
   "misconception": "you ignored the coefficient — one H₂SO₄ holds 7 atoms, and the 2 in front doubles it",
   "feedback": "This answer is wrong — you ignored the coefficient — one H₂SO₄ holds 7 atoms, and the 2 in front doubles it"
  },
  {
   "value": 12,
   "misconception": "you skipped the sulfur — it carries no subscript, so it counts as 1 atom per formula unit",
   "feedback": "This answer is wrong — you skipped the sulfur — it carries no subscript, so it counts as 1 atom per formula unit"
  },
  {
   "value": 9,
   "misconception": "you added the coefficient instead of multiplying by it",
   "feedback": "This answer is wrong — you added the coefficient instead of multiplying by it"
  }
 ]
}

[
 "Write down the values in the question:",
 "mass of MgO = 4.0 g",
 "mass of Mg(NO₃)₂ = 14.8 g",
 "mass of H₂O = 1.8 g",
 "Write down the equation:",
 "total mass of reactants = total mass of products",
 "In this problem's substances:",
 "mass of MgO + mass of HNO₃ = mass of Mg(NO₃)₂ + mass of H₂O",
 "Make the mass of HNO₃ the subject:",
 "mass of HNO₃ = (mass of Mg(NO₃)₂ + mass of H₂O) − mass of MgO",
 "Substitute in the values, and calculate:",
 "mass of HNO₃ = (14.8 + 1.8) − 4.0",
 "mass of HNO₃ = 16.6 − 4.0",
 "mass of HNO₃ = 12.6 g"
]

[
 {
  "value": 20.6,
  "misconception": "you added every mass in the question — the MgO is a reactant, so it is SUBTRACTED from the product total",
  "feedback": "This answer is wrong — you added every mass in the question — the MgO is a reactant, so it is SUBTRACTED from the product total"
 },
 {
  "value": 16.6,
  "misconception": "you totaled the products but forgot to subtract the MgO",
  "feedback": "This answer is wrong — you totaled the products but forgot to subtract the MgO"
 },
 {
  "value": 10.8,
  "misconception": "you ignored the H₂O — both products belong in the product total",
  "feedback": "This answer is wrong — you ignored the H₂O — both products belong in the product total"
 }
]

{
 "keyExplanation": "Write down the values in the question:\nmass of MgO = 4.0 g\nmass of Mg(NO₃)₂ = 14.8 g\nmass of H₂O = 1.8 g\nWrite down the equation:\ntotal mass of reactants = total mass of products\nIn this problem's substances:\nmass of MgO + mass of HNO₃ = mass of Mg(NO₃)₂ + mass of H₂O\nMake the mass of HNO₃ the subject:\nmass of HNO₃ = (mass of Mg(NO₃)₂ + mass of H₂O) − mass of MgO\nSubstitute in the values, and calculate:\nmass of HNO₃ = (14.8 + 1.8) − 4.0\nmass of HNO₃ = 16.6 − 4.0\nmass of HNO₃ = 12.6 g",
 "anticipatedWrong": [
  {
   "value": 20.6,
   "misconception": "you added every mass in the question — the MgO is a reactant, so it is SUBTRACTED from the product total",
   "feedback": "This answer is wrong — you added every mass in the question — the MgO is a reactant, so it is SUBTRACTED from the product total"
  },
  {
   "value": 16.6,
   "misconception": "you totaled the products but forgot to subtract the MgO",
   "feedback": "This answer is wrong — you totaled the products but forgot to subtract the MgO"
  },
  {
   "value": 10.8,
   "misconception": "you ignored the H₂O — both products belong in the product total",
   "feedback": "This answer is wrong — you ignored the H₂O — both products belong in the product total"
  }
 ]
}

[
 {
  "value": "K + I₂ → KI₂",
  "misconception": "you changed the product's subscript to match the iodine — KI₂ is a different substance; KI is fixed by the compound itself, and the fix is coefficients",
  "feedback": "This answer is wrong — you changed the product's subscript to match the iodine — KI₂ is a different substance; KI is fixed by the compound itself, and the fix is coefficients"
 },
 {
  "value": "K + I₂ → 2KI",
  "misconception": "you fixed iodine but never recounted potassium — 2KI leaves potassium at 1 and 2, so K needs a 2 as well",
  "feedback": "This answer is wrong — you fixed iodine but never recounted potassium — 2KI leaves potassium at 1 and 2, so K needs a 2 as well"
 },
 {
  "value": "4K + 2I₂ → 4KI",
  "misconception": "you doubled a set that already worked — use the smallest whole-number coefficients: 2, 1, 2",
  "feedback": "This answer is wrong — you doubled a set that already worked — use the smallest whole-number coefficients: 2, 1, 2"
 }
]

{
 "keyExplanation": "Count: potassium 1 and 1, iodine 2 and 1. Place a 2 in front of KI to fix iodine — that moves potassium to 1 and 2, so place a 2 in front of K. Recount: potassium 2 and 2, iodine 2 and 2 — 2K + I₂ → 2KI.",
 "anticipatedWrong": [
  {
   "value": "K + I₂ → KI₂",
   "misconception": "you changed the product's subscript to match the iodine — KI₂ is a different substance; KI is fixed by the compound itself, and the fix is coefficients",
   "feedback": "This answer is wrong — you changed the product's subscript to match the iodine — KI₂ is a different substance; KI is fixed by the compound itself, and the fix is coefficients"
  },
  {
   "value": "K + I₂ → 2KI",
   "misconception": "you fixed iodine but never recounted potassium — 2KI leaves potassium at 1 and 2, so K needs a 2 as well",
   "feedback": "This answer is wrong — you fixed iodine but never recounted potassium — 2KI leaves potassium at 1 and 2, so K needs a 2 as well"
  },
  {
   "value": "4K + 2I₂ → 4KI",
   "misconception": "you doubled a set that already worked — use the smallest whole-number coefficients: 2, 1, 2",
   "feedback": "This answer is wrong — you doubled a set that already worked — use the smallest whole-number coefficients: 2, 1, 2"
  }
 ]
}

[
 {
  "value": "Sr(NO₃)₂ + K₂CO₃ → SrCO₃ + KNO₃",
  "misconception": "you left everything at 1 — count the units: nitrate is 2 against 1 and potassium 2 against 1; the coefficient 2 on KNO₃ fixes both",
  "feedback": "This answer is wrong — you left everything at 1 — count the units: nitrate is 2 against 1 and potassium 2 against 1; the coefficient 2 on KNO₃ fixes both"
 },
 {
  "value": "Sr(NO₃)₂ + K₂CO₃ → SrCO₃ + K₂(NO₃)₂",
  "misconception": "you changed a formula — K₂(NO₃)₂ is not potassium nitrate; the fix is the coefficient 2 in front of KNO₃, never a new subscript",
  "feedback": "This answer is wrong — you changed a formula — K₂(NO₃)₂ is not potassium nitrate; the fix is the coefficient 2 in front of KNO₃, never a new subscript"
 },
 {
  "value": "Sr(NO₃)₂ + 2K₂CO₃ → SrCO₃ + 2KNO₃",
  "misconception": "you put a 2 on K₂CO₃ as well — recount: potassium is now 4 against 2 and carbonate 2 against 1; only KNO₃ needed the 2",
  "feedback": "This answer is wrong — you put a 2 on K₂CO₃ as well — recount: potassium is now 4 against 2 and carbonate 2 against 1; only KNO₃ needed the 2"
 }
]

{
 "keyExplanation": "Nitrate and carbonate both appear unchanged — count them as units. Count: strontium 1 and 1, nitrate 2 and 1, potassium 2 and 1, carbonate 1 and 1. Place a 2 in front of KNO₃ for nitrate; recount — potassium is now 2 and 2 and carbonate stays 1 and 1, everything matches: Sr(NO₃)₂ + K₂CO₃ → SrCO₃ + 2KNO₃.",
 "anticipatedWrong": [
  {
   "value": "Sr(NO₃)₂ + K₂CO₃ → SrCO₃ + KNO₃",
   "misconception": "you left everything at 1 — count the units: nitrate is 2 against 1 and potassium 2 against 1; the coefficient 2 on KNO₃ fixes both",
   "feedback": "This answer is wrong — you left everything at 1 — count the units: nitrate is 2 against 1 and potassium 2 against 1; the coefficient 2 on KNO₃ fixes both"
  },
  {
   "value": "Sr(NO₃)₂ + K₂CO₃ → SrCO₃ + K₂(NO₃)₂",
   "misconception": "you changed a formula — K₂(NO₃)₂ is not potassium nitrate; the fix is the coefficient 2 in front of KNO₃, never a new subscript",
   "feedback": "This answer is wrong — you changed a formula — K₂(NO₃)₂ is not potassium nitrate; the fix is the coefficient 2 in front of KNO₃, never a new subscript"
  },
  {
   "value": "Sr(NO₃)₂ + 2K₂CO₃ → SrCO₃ + 2KNO₃",
   "misconception": "you put a 2 on K₂CO₃ as well — recount: potassium is now 4 against 2 and carbonate 2 against 1; only KNO₃ needed the 2",
   "feedback": "This answer is wrong — you put a 2 on K₂CO₃ as well — recount: potassium is now 4 against 2 and carbonate 2 against 1; only KNO₃ needed the 2"
  }
 ]
}

[
 {
  "value": "C₅H₁₀ + 7.5O₂ → 5CO₂ + 5H₂O",
  "misconception": "you wrote a fractional coefficient — 7.5 O₂ balances the count, but coefficients must be whole numbers; double everything instead",
  "feedback": "This answer is wrong — you wrote a fractional coefficient — 7.5 O₂ balances the count, but coefficients must be whole numbers; double everything instead"
 },
 {
  "value": "C₅H₁₀ + 7O₂ → 5CO₂ + 5H₂O",
  "misconception": "you rounded the odd oxygen count instead of doubling — 5CO₂ + 5H₂O holds 15 oxygens, and 7O₂ supplies only 14; double every coefficient instead",
  "feedback": "This answer is wrong — you rounded the odd oxygen count instead of doubling — 5CO₂ + 5H₂O holds 15 oxygens, and 7O₂ supplies only 14; double every coefficient instead"
 },
 {
  "value": "C₅H₁₀ + 15O₂ → 5CO₂ + 5H₂O",
  "misconception": "you doubled only the O₂ term — the doubling move multiplies EVERY coefficient, then O₂ is set from the new right side",
  "feedback": "This answer is wrong — you doubled only the O₂ term — the doubling move multiplies EVERY coefficient, then O₂ is set from the new right side"
 }
]

{
 "keyExplanation": "Carbon: 5CO₂. Hydrogen: half of 10 gives 5H₂O. The right side then holds 10 + 5 = 15 oxygen atoms — odd, so double the fuel, CO₂, and H₂O coefficients (O₂ is still unset): 2C₅H₁₀ + O₂ → 10CO₂ + 10H₂O. Oxygen last: 20 + 10 = 30, so 15O₂. Check: carbon 10 and 10, hydrogen 20 and 20, oxygen 30 and 30.",
 "anticipatedWrong": [
  {
   "value": "C₅H₁₀ + 7.5O₂ → 5CO₂ + 5H₂O",
   "misconception": "you wrote a fractional coefficient — 7.5 O₂ balances the count, but coefficients must be whole numbers; double everything instead",
   "feedback": "This answer is wrong — you wrote a fractional coefficient — 7.5 O₂ balances the count, but coefficients must be whole numbers; double everything instead"
  },
  {
   "value": "C₅H₁₀ + 7O₂ → 5CO₂ + 5H₂O",
   "misconception": "you rounded the odd oxygen count instead of doubling — 5CO₂ + 5H₂O holds 15 oxygens, and 7O₂ supplies only 14; double every coefficient instead",
   "feedback": "This answer is wrong — you rounded the odd oxygen count instead of doubling — 5CO₂ + 5H₂O holds 15 oxygens, and 7O₂ supplies only 14; double every coefficient instead"
  },
  {
   "value": "C₅H₁₀ + 15O₂ → 5CO₂ + 5H₂O",
   "misconception": "you doubled only the O₂ term — the doubling move multiplies EVERY coefficient, then O₂ is set from the new right side",
   "feedback": "This answer is wrong — you doubled only the O₂ term — the doubling move multiplies EVERY coefficient, then O₂ is set from the new right side"
  }
 ]
}

[
 {
  "value": "Cu + I₂ → CuI",
  "misconception": "you translated correctly but skipped the balancing — iodine stands at 2 against 1; place a 2 before CuI, then recount copper and place a 2 before Cu",
  "feedback": "This answer is wrong — you translated correctly but skipped the balancing — iodine stands at 2 against 1; place a 2 before CuI, then recount copper and place a 2 before Cu"
 },
 {
  "value": "Cu + I₂ → CuI₂",
  "misconception": "you carried the 2 from I₂ into the product's formula — the (I) says Cu⁺, and Cu⁺ and I⁻ cancel one-to-one, so copper(I) iodide is CuI; subscripts are fixed by the substance, never by the balancing",
  "feedback": "This answer is wrong — you carried the 2 from I₂ into the product's formula — the (I) says Cu⁺, and Cu⁺ and I⁻ cancel one-to-one, so copper(I) iodide is CuI; subscripts are fixed by the substance, never by the balancing"
 },
 {
  "value": "2Cu + 2I → 2CuI",
  "misconception": "you wrote iodine as single atoms — iodine is one of the seven two-atom elements, so uncombined iodine is I₂",
  "feedback": "This answer is wrong — you wrote iodine as single atoms — iodine is one of the seven two-atom elements, so uncombined iodine is I₂"
 }
]

{
 "keyExplanation": "Translate: copper on its own is Cu; iodine uncombined is one of the seven two-atom elements, I₂; copper(I) iodide — the (I) says Cu⁺, and Cu⁺ and I⁻ cancel one-to-one — is CuI. Skeleton: Cu + I₂ → CuI. Balance iodine: place a 2 before CuI; recount — copper is 1 and 2. Balance copper: place a 2 before Cu. Check: 2Cu + I₂ → 2CuI — copper 2 and 2, iodine 2 and 2.",
 "anticipatedWrong": [
  {
   "value": "Cu + I₂ → CuI",
   "misconception": "you translated correctly but skipped the balancing — iodine stands at 2 against 1; place a 2 before CuI, then recount copper and place a 2 before Cu",
   "feedback": "This answer is wrong — you translated correctly but skipped the balancing — iodine stands at 2 against 1; place a 2 before CuI, then recount copper and place a 2 before Cu"
  },
  {
   "value": "Cu + I₂ → CuI₂",
   "misconception": "you carried the 2 from I₂ into the product's formula — the (I) says Cu⁺, and Cu⁺ and I⁻ cancel one-to-one, so copper(I) iodide is CuI; subscripts are fixed by the substance, never by the balancing",
   "feedback": "This answer is wrong — you carried the 2 from I₂ into the product's formula — the (I) says Cu⁺, and Cu⁺ and I⁻ cancel one-to-one, so copper(I) iodide is CuI; subscripts are fixed by the substance, never by the balancing"
  },
  {
   "value": "2Cu + 2I → 2CuI",
   "misconception": "you wrote iodine as single atoms — iodine is one of the seven two-atom elements, so uncombined iodine is I₂",
   "feedback": "This answer is wrong — you wrote iodine as single atoms — iodine is one of the seven two-atom elements, so uncombined iodine is I₂"
  }
 ]
}

[
 {
  "value": "MnSO₄ + BaCl₂ → MnCl₂ + Ba(SO₄)₂",
  "misconception": "you doubled the sulfate — Ba²⁺ and SO₄²⁻ have equal-size charges and cancel one-to-one, so barium sulfate is BaSO₄",
  "feedback": "This answer is wrong — you doubled the sulfate — Ba²⁺ and SO₄²⁻ have equal-size charges and cancel one-to-one, so barium sulfate is BaSO₄"
 },
 {
  "value": "MnSO₄ + BaCl₂ → MnCl + BaSO₄",
  "misconception": "you gave manganese(II) chloride only one chloride — the (II) says Mn²⁺, which takes two Cl⁻: MnCl₂",
  "feedback": "This answer is wrong — you gave manganese(II) chloride only one chloride — the (II) says Mn²⁺, which takes two Cl⁻: MnCl₂"
 }
]

{
 "keyExplanation": "manganese(II) sulfate: the (II) says Mn²⁺, and sulfate is SO₄²⁻ — equal-size charges cancel one-to-one, giving MnSO₄. Barium chloride: Ba²⁺ takes two Cl⁻, giving BaCl₂. Manganese(II) chloride: Mn²⁺ takes two Cl⁻ — MnCl₂. Barium sulfate: Ba²⁺ and SO₄²⁻ cancel one-to-one — BaSO₄. The sulfate ion carries through unchanged. The translation is MnSO₄ + BaCl₂ → MnCl₂ + BaSO₄.",
 "anticipatedWrong": [
  {
   "value": "MnSO₄ + BaCl₂ → MnCl₂ + Ba(SO₄)₂",
   "misconception": "you doubled the sulfate — Ba²⁺ and SO₄²⁻ have equal-size charges and cancel one-to-one, so barium sulfate is BaSO₄",
   "feedback": "This answer is wrong — you doubled the sulfate — Ba²⁺ and SO₄²⁻ have equal-size charges and cancel one-to-one, so barium sulfate is BaSO₄"
  },
  {
   "value": "MnSO₄ + BaCl₂ → MnCl + BaSO₄",
   "misconception": "you gave manganese(II) chloride only one chloride — the (II) says Mn²⁺, which takes two Cl⁻: MnCl₂",
   "feedback": "This answer is wrong — you gave manganese(II) chloride only one chloride — the (II) says Mn²⁺, which takes two Cl⁻: MnCl₂"
  }
 ]
}

[
 {
  "value": 12,
  "misconception": "you ignored the coefficient — one formula unit holds 12 oxygens, and the 4 in front quadruples it",
  "feedback": "This answer is wrong — you ignored the coefficient — one formula unit holds 12 oxygens, and the 4 in front quadruples it"
 },
 {
  "value": 16,
  "misconception": "you skipped the outer subscript 3 — each formula unit holds three SO₄ groups, 12 oxygens, before the coefficient multiplies",
  "feedback": "This answer is wrong — you skipped the outer subscript 3 — each formula unit holds three SO₄ groups, 12 oxygens, before the coefficient multiplies"
 },
 {
  "value": 24,
  "misconception": "you multiplied by the aluminum subscript instead of the coefficient — the 2 counts aluminum atoms only",
  "feedback": "This answer is wrong — you multiplied by the aluminum subscript instead of the coefficient — the 2 counts aluminum atoms only"
 }
]

{
 "keyExplanation": "Count inside one formula unit first: each SO₄ holds 4 oxygens, and the outer subscript 3 makes 3 × 4 = 12. Multiply by the coefficient: 4 × 12 = 48 oxygen atoms.",
 "anticipatedWrong": [
  {
   "value": 12,
   "misconception": "you ignored the coefficient — one formula unit holds 12 oxygens, and the 4 in front quadruples it",
   "feedback": "This answer is wrong — you ignored the coefficient — one formula unit holds 12 oxygens, and the 4 in front quadruples it"
  },
  {
   "value": 16,
   "misconception": "you skipped the outer subscript 3 — each formula unit holds three SO₄ groups, 12 oxygens, before the coefficient multiplies",
   "feedback": "This answer is wrong — you skipped the outer subscript 3 — each formula unit holds three SO₄ groups, 12 oxygens, before the coefficient multiplies"
  },
  {
   "value": 24,
   "misconception": "you multiplied by the aluminum subscript instead of the coefficient — the 2 counts aluminum atoms only",
   "feedback": "This answer is wrong — you multiplied by the aluminum subscript instead of the coefficient — the 2 counts aluminum atoms only"
  }
 ]
}

[
 "Write down the values in the question:",
 "mass of Ca = 4.0 g",
 "mass of S = 3.2 g",
 "Write down the equation:",
 "total mass of reactants = total mass of products",
 "In this problem's substances:",
 "mass of Ca + mass of S = mass of CaS",
 "Make the mass of CaS the subject:",
 "mass of CaS = mass of Ca + mass of S",
 "The mass of CaS is now the subject, so substitute in the values, and calculate:",
 "mass of CaS = 4.0 + 3.2",
 "mass of CaS = 7.2 g"
]

[
 {
  "value": 0.8,
  "misconception": "you subtracted the reactant masses — BOTH reactants end up inside the CaS, so add them",
  "feedback": "This answer is wrong — you subtracted the reactant masses — BOTH reactants end up inside the CaS, so add them"
 },
 {
  "value": 4.0,
  "misconception": "you counted only the Ca — the S's 3.2 g also ends up in the CaS",
  "feedback": "This answer is wrong — you counted only the Ca — the S's 3.2 g also ends up in the CaS"
 },
 {
  "value": 3.2,
  "misconception": "you counted only the S — the Ca's 4.0 g also ends up in the CaS",
  "feedback": "This answer is wrong — you counted only the S — the Ca's 4.0 g also ends up in the CaS"
 }
]

{
 "keyExplanation": "Write down the values in the question:\nmass of Ca = 4.0 g\nmass of S = 3.2 g\nWrite down the equation:\ntotal mass of reactants = total mass of products\nIn this problem's substances:\nmass of Ca + mass of S = mass of CaS\nMake the mass of CaS the subject:\nmass of CaS = mass of Ca + mass of S\nThe mass of CaS is now the subject, so substitute in the values, and calculate:\nmass of CaS = 4.0 + 3.2\nmass of CaS = 7.2 g",
 "anticipatedWrong": [
  {
   "value": 0.8,
   "misconception": "you subtracted the reactant masses — BOTH reactants end up inside the CaS, so add them",
   "feedback": "This answer is wrong — you subtracted the reactant masses — BOTH reactants end up inside the CaS, so add them"
  },
  {
   "value": 4.0,
   "misconception": "you counted only the Ca — the S's 3.2 g also ends up in the CaS",
   "feedback": "This answer is wrong — you counted only the Ca — the S's 3.2 g also ends up in the CaS"
  },
  {
   "value": 3.2,
   "misconception": "you counted only the S — the Ca's 4.0 g also ends up in the CaS",
   "feedback": "This answer is wrong — you counted only the S — the Ca's 4.0 g also ends up in the CaS"
  }
 ]
}

[
 {
  "value": "Al + I₂ → AlI₂",
  "misconception": "you rewrote the product to match the diatomic — AlI₂ is a different substance; AlI₃ is fixed by the compound itself, and the fix is coefficients",
  "feedback": "This answer is wrong — you rewrote the product to match the diatomic — AlI₂ is a different substance; AlI₃ is fixed by the compound itself, and the fix is coefficients"
 },
 {
  "value": "Al + 3I₂ → 2AlI₃",
  "misconception": "you fixed iodine but never recounted aluminum — 2AlI₃ leaves aluminum at 1 and 2, so Al needs a 2 as well",
  "feedback": "This answer is wrong — you fixed iodine but never recounted aluminum — 2AlI₃ leaves aluminum at 1 and 2, so Al needs a 2 as well"
 },
 {
  "value": "4Al + 6I₂ → 4AlI₃",
  "misconception": "you doubled a set that already worked — use the smallest whole-number coefficients: 2, 3, 2",
  "feedback": "This answer is wrong — you doubled a set that already worked — use the smallest whole-number coefficients: 2, 3, 2"
 }
]

{
 "keyExplanation": "Count: aluminum 1 and 1, iodine 2 and 3. The iodine counts meet at 6: place a 3 in front of I₂ and a 2 in front of AlI₃ — that moves aluminum to 1 and 2, so place a 2 in front of Al. Recount: aluminum 2 and 2, iodine 6 and 6 — 2Al + 3I₂ → 2AlI₃.",
 "anticipatedWrong": [
  {
   "value": "Al + I₂ → AlI₂",
   "misconception": "you rewrote the product to match the diatomic — AlI₂ is a different substance; AlI₃ is fixed by the compound itself, and the fix is coefficients",
   "feedback": "This answer is wrong — you rewrote the product to match the diatomic — AlI₂ is a different substance; AlI₃ is fixed by the compound itself, and the fix is coefficients"
  },
  {
   "value": "Al + 3I₂ → 2AlI₃",
   "misconception": "you fixed iodine but never recounted aluminum — 2AlI₃ leaves aluminum at 1 and 2, so Al needs a 2 as well",
   "feedback": "This answer is wrong — you fixed iodine but never recounted aluminum — 2AlI₃ leaves aluminum at 1 and 2, so Al needs a 2 as well"
  },
  {
   "value": "4Al + 6I₂ → 4AlI₃",
   "misconception": "you doubled a set that already worked — use the smallest whole-number coefficients: 2, 3, 2",
   "feedback": "This answer is wrong — you doubled a set that already worked — use the smallest whole-number coefficients: 2, 3, 2"
  }
 ]
}

[
 {
  "value": "Ba(OH)₂ + Li₂SO₄ → BaSO₄ + LiOH",
  "misconception": "you left the counts at 1 — hydroxide is 2 against 1 and lithium 2 against 1; the coefficient 2 on LiOH fixes both",
  "feedback": "This answer is wrong — you left the counts at 1 — hydroxide is 2 against 1 and lithium 2 against 1; the coefficient 2 on LiOH fixes both"
 },
 {
  "value": "Ba(OH)₂ + Li₂SO₄ → BaSO₄ + Li(OH)₂",
  "misconception": "you changed a formula — Li(OH)₂ does not exist; hydroxide's fix is the coefficient 2 in front of LiOH",
  "feedback": "This answer is wrong — you changed a formula — Li(OH)₂ does not exist; hydroxide's fix is the coefficient 2 in front of LiOH"
 },
 {
  "value": "2Ba(OH)₂ + Li₂SO₄ → BaSO₄ + 2LiOH",
  "misconception": "you doubled Ba(OH)₂ as well — recount: barium is now 2 against 1 and hydroxide 4 against 2; only LiOH needed the 2",
  "feedback": "This answer is wrong — you doubled Ba(OH)₂ as well — recount: barium is now 2 against 1 and hydroxide 4 against 2; only LiOH needed the 2"
 }
]

{
 "keyExplanation": "Hydroxide and sulfate both appear unchanged — count them as units. Count: barium 1 and 1, hydroxide 2 and 1, lithium 2 and 1, sulfate 1 and 1. Place a 2 in front of LiOH for hydroxide; recount — lithium is now 2 and 2 and sulfate stays 1 and 1, everything matches: Ba(OH)₂ + Li₂SO₄ → BaSO₄ + 2LiOH.",
 "anticipatedWrong": [
  {
   "value": "Ba(OH)₂ + Li₂SO₄ → BaSO₄ + LiOH",
   "misconception": "you left the counts at 1 — hydroxide is 2 against 1 and lithium 2 against 1; the coefficient 2 on LiOH fixes both",
   "feedback": "This answer is wrong — you left the counts at 1 — hydroxide is 2 against 1 and lithium 2 against 1; the coefficient 2 on LiOH fixes both"
  },
  {
   "value": "Ba(OH)₂ + Li₂SO₄ → BaSO₄ + Li(OH)₂",
   "misconception": "you changed a formula — Li(OH)₂ does not exist; hydroxide's fix is the coefficient 2 in front of LiOH",
   "feedback": "This answer is wrong — you changed a formula — Li(OH)₂ does not exist; hydroxide's fix is the coefficient 2 in front of LiOH"
  },
  {
   "value": "2Ba(OH)₂ + Li₂SO₄ → BaSO₄ + 2LiOH",
   "misconception": "you doubled Ba(OH)₂ as well — recount: barium is now 2 against 1 and hydroxide 4 against 2; only LiOH needed the 2",
   "feedback": "This answer is wrong — you doubled Ba(OH)₂ as well — recount: barium is now 2 against 1 and hydroxide 4 against 2; only LiOH needed the 2"
  }
 ]
}

[
 {
  "value": "C₆H₆ + 8O₂ → 6CO₂ + 3H₂O",
  "misconception": "you rounded the odd oxygen count instead of doubling — 6CO₂ + 3H₂O holds 15 oxygens, and 8O₂ would supply 16; double every coefficient instead",
  "feedback": "This answer is wrong — you rounded the odd oxygen count instead of doubling — 6CO₂ + 3H₂O holds 15 oxygens, and 8O₂ would supply 16; double every coefficient instead"
 },
 {
  "value": "C₆H₆ + 7.5O₂ → 6CO₂ + 3H₂O",
  "misconception": "you wrote a fractional coefficient — 7.5 O₂ balances the count, but coefficients must be whole numbers; double everything instead",
  "feedback": "This answer is wrong — you wrote a fractional coefficient — 7.5 O₂ balances the count, but coefficients must be whole numbers; double everything instead"
 },
 {
  "value": "2C₆H₆ + 15O₂ → 12CO₂ + 3H₂O",
  "misconception": "you halved the hydrogen twice — 2C₆H₆ carries 12 hydrogens, needing 6H₂O, not 3",
  "feedback": "This answer is wrong — you halved the hydrogen twice — 2C₆H₆ carries 12 hydrogens, needing 6H₂O, not 3"
 }
]

{
 "keyExplanation": "Carbon: 6CO₂. Hydrogen: half of 6 gives 3H₂O. The right side then holds 12 + 3 = 15 oxygen atoms — odd, so double the fuel, CO₂, and H₂O coefficients (O₂ is still unset): 2C₆H₆ + O₂ → 12CO₂ + 6H₂O. Oxygen last: 24 + 6 = 30, so 15O₂. Check: carbon 12 and 12, hydrogen 12 and 12, oxygen 30 and 30.",
 "anticipatedWrong": [
  {
   "value": "C₆H₆ + 8O₂ → 6CO₂ + 3H₂O",
   "misconception": "you rounded the odd oxygen count instead of doubling — 6CO₂ + 3H₂O holds 15 oxygens, and 8O₂ would supply 16; double every coefficient instead",
   "feedback": "This answer is wrong — you rounded the odd oxygen count instead of doubling — 6CO₂ + 3H₂O holds 15 oxygens, and 8O₂ would supply 16; double every coefficient instead"
  },
  {
   "value": "C₆H₆ + 7.5O₂ → 6CO₂ + 3H₂O",
   "misconception": "you wrote a fractional coefficient — 7.5 O₂ balances the count, but coefficients must be whole numbers; double everything instead",
   "feedback": "This answer is wrong — you wrote a fractional coefficient — 7.5 O₂ balances the count, but coefficients must be whole numbers; double everything instead"
  },
  {
   "value": "2C₆H₆ + 15O₂ → 12CO₂ + 3H₂O",
   "misconception": "you halved the hydrogen twice — 2C₆H₆ carries 12 hydrogens, needing 6H₂O, not 3",
   "feedback": "This answer is wrong — you halved the hydrogen twice — 2C₆H₆ carries 12 hydrogens, needing 6H₂O, not 3"
  }
 ]
}

[
 {
  "value": "Ba(NO₃)₂ + K₂CO₃ → BaCO₃ + KNO₃",
  "misconception": "you translated correctly but skipped the balancing — potassium and nitrate both stand at 2 against 1, and one 2 before KNO₃ fixes both",
  "feedback": "This answer is wrong — you translated correctly but skipped the balancing — potassium and nitrate both stand at 2 against 1, and one 2 before KNO₃ fixes both"
 },
 {
  "value": "Ba(NO₃)₂ + K₂CO₃ → BaCO₃ + K₂NO₃",
  "misconception": "you changed a formula — K₂NO₃ does not exist; potassium nitrate, from K⁺ and NO₃⁻, is KNO₃, and the fix is the coefficient 2",
  "feedback": "This answer is wrong — you changed a formula — K₂NO₃ does not exist; potassium nitrate, from K⁺ and NO₃⁻, is KNO₃, and the fix is the coefficient 2"
 },
 {
  "value": "Ba(NO₃)₂ + K₂CO₃ → Ba(CO₃)₂ + 2KNO₃",
  "misconception": "you gave barium carbonate a doubled formula — from Ba²⁺ and CO₃²⁻ the charges cancel one to one, so the product is BaCO₃",
  "feedback": "This answer is wrong — you gave barium carbonate a doubled formula — from Ba²⁺ and CO₃²⁻ the charges cancel one to one, so the product is BaCO₃"
 }
]

{
 "keyExplanation": "Translate: barium nitrate — Ba²⁺ with two NO₃⁻ — is Ba(NO₃)₂; potassium carbonate — two K⁺ with CO₃²⁻ — is K₂CO₃; barium carbonate — the 2+ and 2− charges cancel one-to-one — is BaCO₃; potassium nitrate is KNO₃. Skeleton: Ba(NO₃)₂ + K₂CO₃ → BaCO₃ + KNO₃. Nitrate and carbonate appear unchanged — count them as units: barium 1 and 1, carbonate 1 and 1, nitrate 2 and 1, potassium 2 and 1. Place a 2 before KNO₃. Check: Ba(NO₃)₂ + K₂CO₃ → BaCO₃ + 2KNO₃ — every count matches.",
 "anticipatedWrong": [
  {
   "value": "Ba(NO₃)₂ + K₂CO₃ → BaCO₃ + KNO₃",
   "misconception": "you translated correctly but skipped the balancing — potassium and nitrate both stand at 2 against 1, and one 2 before KNO₃ fixes both",
   "feedback": "This answer is wrong — you translated correctly but skipped the balancing — potassium and nitrate both stand at 2 against 1, and one 2 before KNO₃ fixes both"
  },
  {
   "value": "Ba(NO₃)₂ + K₂CO₃ → BaCO₃ + K₂NO₃",
   "misconception": "you changed a formula — K₂NO₃ does not exist; potassium nitrate, from K⁺ and NO₃⁻, is KNO₃, and the fix is the coefficient 2",
   "feedback": "This answer is wrong — you changed a formula — K₂NO₃ does not exist; potassium nitrate, from K⁺ and NO₃⁻, is KNO₃, and the fix is the coefficient 2"
  },
  {
   "value": "Ba(NO₃)₂ + K₂CO₃ → Ba(CO₃)₂ + 2KNO₃",
   "misconception": "you gave barium carbonate a doubled formula — from Ba²⁺ and CO₃²⁻ the charges cancel one to one, so the product is BaCO₃",
   "feedback": "This answer is wrong — you gave barium carbonate a doubled formula — from Ba²⁺ and CO₃²⁻ the charges cancel one to one, so the product is BaCO₃"
  }
 ]
}

[
 {
  "value": "Ni₂SO₄ + BaCl₂ → NiCl₂ + BaSO₄",
  "misconception": "you read the (II) as a count of nickel atoms — the Roman numeral is the charge on the nickel ion (Ni²⁺), and Ni²⁺ cancels SO₄²⁻ one-to-one: NiSO₄",
  "feedback": "This answer is wrong — you read the (II) as a count of nickel atoms — the Roman numeral is the charge on the nickel ion (Ni²⁺), and Ni²⁺ cancels SO₄²⁻ one-to-one: NiSO₄"
 },
 {
  "value": "NiSO₃ + BaCl₂ → NiCl₂ + BaSO₃",
  "misconception": "you wrote sulfite instead of sulfate — sulfate is the polyatomic ion SO₄²⁻; SO₃²⁻ is sulfite",
  "feedback": "This answer is wrong — you wrote sulfite instead of sulfate — sulfate is the polyatomic ion SO₄²⁻; SO₃²⁻ is sulfite"
 }
]

{
 "keyExplanation": "nickel(II) sulfate: the (II) says Ni²⁺, and sulfate is SO₄²⁻ — equal-size charges cancel one-to-one, giving NiSO₄. Barium chloride: Ba²⁺ takes two Cl⁻, giving BaCl₂. Nickel(II) chloride: Ni²⁺ takes two Cl⁻ — NiCl₂. Barium sulfate: Ba²⁺ and SO₄²⁻ cancel one-to-one — BaSO₄. The sulfate ion carries through unchanged. The translation is NiSO₄ + BaCl₂ → NiCl₂ + BaSO₄.",
 "anticipatedWrong": [
  {
   "value": "Ni₂SO₄ + BaCl₂ → NiCl₂ + BaSO₄",
   "misconception": "you read the (II) as a count of nickel atoms — the Roman numeral is the charge on the nickel ion (Ni²⁺), and Ni²⁺ cancels SO₄²⁻ one-to-one: NiSO₄",
   "feedback": "This answer is wrong — you read the (II) as a count of nickel atoms — the Roman numeral is the charge on the nickel ion (Ni²⁺), and Ni²⁺ cancels SO₄²⁻ one-to-one: NiSO₄"
  },
  {
   "value": "NiSO₃ + BaCl₂ → NiCl₂ + BaSO₃",
   "misconception": "you wrote sulfite instead of sulfate — sulfate is the polyatomic ion SO₄²⁻; SO₃²⁻ is sulfite",
   "feedback": "This answer is wrong — you wrote sulfite instead of sulfate — sulfate is the polyatomic ion SO₄²⁻; SO₃²⁻ is sulfite"
  }
 ]
}

[
 {
  "value": 5,
  "misconception": "you ignored the coefficient — one KNO₃ holds 5 atoms, and the 3 in front triples it",
  "feedback": "This answer is wrong — you ignored the coefficient — one KNO₃ holds 5 atoms, and the 3 in front triples it"
 },
 {
  "value": 9,
  "misconception": "you skipped the potassium and the nitrogen — each carries no subscript, so each counts as 1 atom per formula unit",
  "feedback": "This answer is wrong — you skipped the potassium and the nitrogen — each carries no subscript, so each counts as 1 atom per formula unit"
 },
 {
  "value": 8,
  "misconception": "you added the coefficient instead of multiplying by it",
  "feedback": "This answer is wrong — you added the coefficient instead of multiplying by it"
 }
]

{
 "keyExplanation": "Count one formula unit: 1 potassium + 1 nitrogen + 3 oxygens = 5 atoms. Multiply by the coefficient: 3 × 5 = 15 atoms in total.",
 "anticipatedWrong": [
  {
   "value": 5,
   "misconception": "you ignored the coefficient — one KNO₃ holds 5 atoms, and the 3 in front triples it",
   "feedback": "This answer is wrong — you ignored the coefficient — one KNO₃ holds 5 atoms, and the 3 in front triples it"
  },
  {
   "value": 9,
   "misconception": "you skipped the potassium and the nitrogen — each carries no subscript, so each counts as 1 atom per formula unit",
   "feedback": "This answer is wrong — you skipped the potassium and the nitrogen — each carries no subscript, so each counts as 1 atom per formula unit"
  },
  {
   "value": 8,
   "misconception": "you added the coefficient instead of multiplying by it",
   "feedback": "This answer is wrong — you added the coefficient instead of multiplying by it"
  }
 ]
}

[
 "Write down the values in the question:",
 "mass of ZnCO₃ = 12.5 g",
 "mass of ZnO = 8.1 g",
 "Write down the equation:",
 "total mass of reactants = total mass of products",
 "In this problem's substances:",
 "mass of ZnCO₃ = mass of ZnO + mass of CO₂",
 "Make the mass of CO₂ the subject:",
 "mass of CO₂ = mass of ZnCO₃ − mass of ZnO",
 "Substitute in the values, and calculate:",
 "mass of CO₂ = 12.5 − 8.1",
 "mass of CO₂ = 4.4 g"
]

[
 {
  "value": 20.6,
  "misconception": "you added the two masses — the ZnO is what remains of the 12.5 g, so subtract it",
  "feedback": "This answer is wrong — you added the two masses — the ZnO is what remains of the 12.5 g, so subtract it"
 },
 {
  "value": 12.5,
  "misconception": "you repeated the starting mass — the 12.5 g splits between the ZnO and the CO₂, so subtract the ZnO",
  "feedback": "This answer is wrong — you repeated the starting mass — the 12.5 g splits between the ZnO and the CO₂, so subtract the ZnO"
 },
 {
  "value": 8.1,
  "misconception": "you gave the mass of the leftover solid — the question asks for the gas that formed",
  "feedback": "This answer is wrong — you gave the mass of the leftover solid — the question asks for the gas that formed"
 }
]

{
 "keyExplanation": "Write down the values in the question:\nmass of ZnCO₃ = 12.5 g\nmass of ZnO = 8.1 g\nWrite down the equation:\ntotal mass of reactants = total mass of products\nIn this problem's substances:\nmass of ZnCO₃ = mass of ZnO + mass of CO₂\nMake the mass of CO₂ the subject:\nmass of CO₂ = mass of ZnCO₃ − mass of ZnO\nSubstitute in the values, and calculate:\nmass of CO₂ = 12.5 − 8.1\nmass of CO₂ = 4.4 g",
 "anticipatedWrong": [
  {
   "value": 20.6,
   "misconception": "you added the two masses — the ZnO is what remains of the 12.5 g, so subtract it",
   "feedback": "This answer is wrong — you added the two masses — the ZnO is what remains of the 12.5 g, so subtract it"
  },
  {
   "value": 12.5,
   "misconception": "you repeated the starting mass — the 12.5 g splits between the ZnO and the CO₂, so subtract the ZnO",
   "feedback": "This answer is wrong — you repeated the starting mass — the 12.5 g splits between the ZnO and the CO₂, so subtract the ZnO"
  },
  {
   "value": 8.1,
   "misconception": "you gave the mass of the leftover solid — the question asks for the gas that formed",
   "feedback": "This answer is wrong — you gave the mass of the leftover solid — the question asks for the gas that formed"
  }
 ]
}

[
 {
  "value": "Li + N₂ → Li₃N₂",
  "misconception": "you changed the product's subscript to match the nitrogen — Li₃N is fixed by the compound itself; the fix is coefficients",
  "feedback": "This answer is wrong — you changed the product's subscript to match the nitrogen — Li₃N is fixed by the compound itself; the fix is coefficients"
 },
 {
  "value": "3Li + N₂ → Li₃N",
  "misconception": "you fixed lithium but never recounted nitrogen — 3Li and Li₃N match at 3 and 3, but nitrogen still sits at 2 and 1",
  "feedback": "This answer is wrong — you fixed lithium but never recounted nitrogen — 3Li and Li₃N match at 3 and 3, but nitrogen still sits at 2 and 1"
 },
 {
  "value": "12Li + 2N₂ → 4Li₃N",
  "misconception": "you doubled a set that already worked — use the smallest whole-number coefficients: 6, 1, 2",
  "feedback": "This answer is wrong — you doubled a set that already worked — use the smallest whole-number coefficients: 6, 1, 2"
 }
]

{
 "keyExplanation": "Count: lithium 1 and 3, nitrogen 2 and 1. Place a 2 in front of Li₃N to fix nitrogen — that moves lithium to 1 and 6, so place a 6 in front of Li. Recount: lithium 6 and 6, nitrogen 2 and 2 — 6Li + N₂ → 2Li₃N.",
 "anticipatedWrong": [
  {
   "value": "Li + N₂ → Li₃N₂",
   "misconception": "you changed the product's subscript to match the nitrogen — Li₃N is fixed by the compound itself; the fix is coefficients",
   "feedback": "This answer is wrong — you changed the product's subscript to match the nitrogen — Li₃N is fixed by the compound itself; the fix is coefficients"
  },
  {
   "value": "3Li + N₂ → Li₃N",
   "misconception": "you fixed lithium but never recounted nitrogen — 3Li and Li₃N match at 3 and 3, but nitrogen still sits at 2 and 1",
   "feedback": "This answer is wrong — you fixed lithium but never recounted nitrogen — 3Li and Li₃N match at 3 and 3, but nitrogen still sits at 2 and 1"
  },
  {
   "value": "12Li + 2N₂ → 4Li₃N",
   "misconception": "you doubled a set that already worked — use the smallest whole-number coefficients: 6, 1, 2",
   "feedback": "This answer is wrong — you doubled a set that already worked — use the smallest whole-number coefficients: 6, 1, 2"
  }
 ]
}

[
 {
  "value": "AlCl₃ + K₃PO₄ → AlPO₄ + KCl",
  "misconception": "you left everything at 1 — chlorine is 3 against 1 and potassium 3 against 1; the coefficient 3 on KCl fixes both",
  "feedback": "This answer is wrong — you left everything at 1 — chlorine is 3 against 1 and potassium 3 against 1; the coefficient 3 on KCl fixes both"
 },
 {
  "value": "AlCl₃ + K₃PO₄ → AlPO₄ + K₃Cl₃",
  "misconception": "you changed a formula — K₃Cl₃ is not potassium chloride; the fix is the coefficient 3 in front of KCl, never a new subscript",
  "feedback": "This answer is wrong — you changed a formula — K₃Cl₃ is not potassium chloride; the fix is the coefficient 3 in front of KCl, never a new subscript"
 },
 {
  "value": "3AlCl₃ + K₃PO₄ → AlPO₄ + 3KCl",
  "misconception": "you matched the 3 on the left as well — recount: aluminum is now 3 against 1 and chlorine 9 against 3; only KCl needed the 3",
  "feedback": "This answer is wrong — you matched the 3 on the left as well — recount: aluminum is now 3 against 1 and chlorine 9 against 3; only KCl needed the 3"
 }
]

{
 "keyExplanation": "Phosphate appears unchanged on both sides — count it as a unit. Count: aluminum 1 and 1, chlorine 3 and 1, potassium 3 and 1, phosphate 1 and 1. Place a 3 in front of KCl for chlorine; recount — potassium is now 3 and 3 and phosphate stays 1 and 1, everything matches: AlCl₃ + K₃PO₄ → AlPO₄ + 3KCl.",
 "anticipatedWrong": [
  {
   "value": "AlCl₃ + K₃PO₄ → AlPO₄ + KCl",
   "misconception": "you left everything at 1 — chlorine is 3 against 1 and potassium 3 against 1; the coefficient 3 on KCl fixes both",
   "feedback": "This answer is wrong — you left everything at 1 — chlorine is 3 against 1 and potassium 3 against 1; the coefficient 3 on KCl fixes both"
  },
  {
   "value": "AlCl₃ + K₃PO₄ → AlPO₄ + K₃Cl₃",
   "misconception": "you changed a formula — K₃Cl₃ is not potassium chloride; the fix is the coefficient 3 in front of KCl, never a new subscript",
   "feedback": "This answer is wrong — you changed a formula — K₃Cl₃ is not potassium chloride; the fix is the coefficient 3 in front of KCl, never a new subscript"
  },
  {
   "value": "3AlCl₃ + K₃PO₄ → AlPO₄ + 3KCl",
   "misconception": "you matched the 3 on the left as well — recount: aluminum is now 3 against 1 and chlorine 9 against 3; only KCl needed the 3",
   "feedback": "This answer is wrong — you matched the 3 on the left as well — recount: aluminum is now 3 against 1 and chlorine 9 against 3; only KCl needed the 3"
  }
 ]
}

[
 {
  "value": "C₁₀H₂₂ + 15O₂ → 10CO₂ + 11H₂O",
  "misconception": "you left the odd oxygen count standing — 10CO₂ + 11H₂O holds 31 oxygens, and no whole O₂ coefficient supplies 31; double every coefficient first",
  "feedback": "This answer is wrong — you left the odd oxygen count standing — 10CO₂ + 11H₂O holds 31 oxygens, and no whole O₂ coefficient supplies 31; double every coefficient first"
 },
 {
  "value": "C₁₀H₂₂ + 31O₂ → 10CO₂ + 11H₂O",
  "misconception": "you doubled only the O₂ term — the doubling move multiplies EVERY coefficient, then O₂ is set from the new right side",
  "feedback": "This answer is wrong — you doubled only the O₂ term — the doubling move multiplies EVERY coefficient, then O₂ is set from the new right side"
 },
 {
  "value": "2C₁₀H₂₂ + 31O₂ → 20CO₂ + 11H₂O",
  "misconception": "you halved the hydrogen twice — 2C₁₀H₂₂ carries 44 hydrogens, needing 22H₂O, not 11",
  "feedback": "This answer is wrong — you halved the hydrogen twice — 2C₁₀H₂₂ carries 44 hydrogens, needing 22H₂O, not 11"
 }
]

{
 "keyExplanation": "Carbon: 10CO₂. Hydrogen: half of 22 gives 11H₂O. The right side then holds 20 + 11 = 31 oxygen atoms — odd, so double the fuel, CO₂, and H₂O coefficients (O₂ is still unset): 2C₁₀H₂₂ + O₂ → 20CO₂ + 22H₂O. Oxygen last: 40 + 22 = 62, so 31O₂. Check: carbon 20 and 20, hydrogen 44 and 44, oxygen 62 and 62.",
 "anticipatedWrong": [
  {
   "value": "C₁₀H₂₂ + 15O₂ → 10CO₂ + 11H₂O",
   "misconception": "you left the odd oxygen count standing — 10CO₂ + 11H₂O holds 31 oxygens, and no whole O₂ coefficient supplies 31; double every coefficient first",
   "feedback": "This answer is wrong — you left the odd oxygen count standing — 10CO₂ + 11H₂O holds 31 oxygens, and no whole O₂ coefficient supplies 31; double every coefficient first"
  },
  {
   "value": "C₁₀H₂₂ + 31O₂ → 10CO₂ + 11H₂O",
   "misconception": "you doubled only the O₂ term — the doubling move multiplies EVERY coefficient, then O₂ is set from the new right side",
   "feedback": "This answer is wrong — you doubled only the O₂ term — the doubling move multiplies EVERY coefficient, then O₂ is set from the new right side"
  },
  {
   "value": "2C₁₀H₂₂ + 31O₂ → 20CO₂ + 11H₂O",
   "misconception": "you halved the hydrogen twice — 2C₁₀H₂₂ carries 44 hydrogens, needing 22H₂O, not 11",
   "feedback": "This answer is wrong — you halved the hydrogen twice — 2C₁₀H₂₂ carries 44 hydrogens, needing 22H₂O, not 11"
  }
 ]
}

[
 {
  "value": "Cr(NO₃)₃ + KOH → Cr(OH)₃ + KNO₃",
  "misconception": "you translated correctly but skipped the balancing — count hydroxide as a unit: it stands at 3 against 1, and a 3 before KOH plus a 3 before KNO₃ make every count match",
  "feedback": "This answer is wrong — you translated correctly but skipped the balancing — count hydroxide as a unit: it stands at 3 against 1, and a 3 before KOH plus a 3 before KNO₃ make every count match"
 },
 {
  "value": "CrNO₃ + KOH → CrOH + KNO₃",
  "misconception": "you built the formulas one-to-one instead of from the ion charges — the (III) says Cr³⁺, so chromium(III) nitrate is Cr(NO₃)₃ and chromium(III) hydroxide is Cr(OH)₃",
  "feedback": "This answer is wrong — you built the formulas one-to-one instead of from the ion charges — the (III) says Cr³⁺, so chromium(III) nitrate is Cr(NO₃)₃ and chromium(III) hydroxide is Cr(OH)₃"
 },
 {
  "value": "Cr(NO₃)₃ + 3KOH → Cr(OH)₃ + K₃NO₃",
  "misconception": "you changed a formula to absorb the extra potassium — K₃NO₃ does not exist; potassium nitrate is KNO₃, and the fix is the coefficient 3",
  "feedback": "This answer is wrong — you changed a formula to absorb the extra potassium — K₃NO₃ does not exist; potassium nitrate is KNO₃, and the fix is the coefficient 3"
 }
]

{
 "keyExplanation": "Translate: chromium(III) nitrate — the (III) says Cr³⁺, with three NO₃⁻ — is Cr(NO₃)₃; potassium hydroxide is KOH; chromium(III) hydroxide is Cr(OH)₃; potassium nitrate is KNO₃. Skeleton: Cr(NO₃)₃ + KOH → Cr(OH)₃ + KNO₃. Hydroxide and nitrate appear unchanged — count them as units: hydroxide is 1 and 3, so place a 3 before KOH; recount — potassium is 3 and 1 and nitrate is 3 and 1, so place a 3 before KNO₃. Check: Cr(NO₃)₃ + 3KOH → Cr(OH)₃ + 3KNO₃ — chromium 1 and 1, nitrate 3 and 3, potassium 3 and 3, hydroxide 3 and 3.",
 "anticipatedWrong": [
  {
   "value": "Cr(NO₃)₃ + KOH → Cr(OH)₃ + KNO₃",
   "misconception": "you translated correctly but skipped the balancing — count hydroxide as a unit: it stands at 3 against 1, and a 3 before KOH plus a 3 before KNO₃ make every count match",
   "feedback": "This answer is wrong — you translated correctly but skipped the balancing — count hydroxide as a unit: it stands at 3 against 1, and a 3 before KOH plus a 3 before KNO₃ make every count match"
  },
  {
   "value": "CrNO₃ + KOH → CrOH + KNO₃",
   "misconception": "you built the formulas one-to-one instead of from the ion charges — the (III) says Cr³⁺, so chromium(III) nitrate is Cr(NO₃)₃ and chromium(III) hydroxide is Cr(OH)₃",
   "feedback": "This answer is wrong — you built the formulas one-to-one instead of from the ion charges — the (III) says Cr³⁺, so chromium(III) nitrate is Cr(NO₃)₃ and chromium(III) hydroxide is Cr(OH)₃"
  },
  {
   "value": "Cr(NO₃)₃ + 3KOH → Cr(OH)₃ + K₃NO₃",
   "misconception": "you changed a formula to absorb the extra potassium — K₃NO₃ does not exist; potassium nitrate is KNO₃, and the fix is the coefficient 3",
   "feedback": "This answer is wrong — you changed a formula to absorb the extra potassium — K₃NO₃ does not exist; potassium nitrate is KNO₃, and the fix is the coefficient 3"
  }
 ]
}

{
 "keyExplanation": "Charges from position: barium is Group 2, so Ba²⁺; nitrogen is Group 15, so N³⁻. Criss-cross: the 2 crosses to N, the 3 crosses to Ba, giving Ba₃N₂. Check: 3 × (2+) + 2 × (3−) = 0.",
 "incorrectExplanation": "If you got Ba₂N₃: you criss-crossed in reverse — nitrogen's 3 becomes BARIUM's subscript and barium's 2 becomes NITROGEN's subscript. If you got BaN: you ignored the charges — a 2+ ion cannot balance a 3− ion one to one; the charge numbers must criss-cross. If you got BaN₂: you kept nitrogen's molecular subscript — the product is built from ions, Ba²⁺ and N³⁻, not from N₂ molecules."
}

[
 {
  "value": "Al(NO₃)₂",
  "misconception": "you copied the old compound's subscript instead of re-crossing the charges",
  "feedback": "This answer is wrong — the 2 belonged to lead's compound. Re-cross Al³⁺ with NO₃⁻: Al(NO₃)₃."
 },
 {
  "value": "AlNO₃",
  "misconception": "you ignored the charges and paired one-to-one",
  "feedback": "This answer is wrong — Al³⁺ needs three nitrate partners: Al(NO₃)₃."
 },
 {
  "value": "Al₃NO₃",
  "misconception": "you crossed the charges in the reverse direction",
  "feedback": "This answer is wrong — the 3 from Al³⁺ crosses to the nitrate: Al(NO₃)₃."
 }
]

{
 "keyExplanation": "Aluminum takes lead's place and pairs with the nitrate ions. Al³⁺ and NO₃⁻ criss-cross to Al(NO₃)₃ — never copy the old compound's subscripts.",
 "anticipatedWrong": [
  {
   "value": "Al(NO₃)₂",
   "misconception": "you copied the old compound's subscript instead of re-crossing the charges",
   "feedback": "This answer is wrong — the 2 belonged to lead's compound. Re-cross Al³⁺ with NO₃⁻: Al(NO₃)₃."
  },
  {
   "value": "AlNO₃",
   "misconception": "you ignored the charges and paired one-to-one",
   "feedback": "This answer is wrong — Al³⁺ needs three nitrate partners: Al(NO₃)₃."
  },
  {
   "value": "Al₃NO₃",
   "misconception": "you crossed the charges in the reverse direction",
   "feedback": "This answer is wrong — the 3 from Al³⁺ crosses to the nitrate: Al(NO₃)₃."
  }
 ]
}

{
 "keyExplanation": "Barium's new partner is sulfate. Ba²⁺ and SO₄²⁻ carry equal charge numbers, so they balance one to one: BaSO₄. The full equation is K₂SO₄ + Ba(OH)₂ → BaSO₄ + 2KOH.",
 "incorrectExplanation": "If you got Ba(SO₄)₂: you copied the old hydroxide subscript onto the sulfate — Ba²⁺ and SO₄²⁻ balance one to one. If you got Ba₂SO₄: you copied potassium's subscript onto barium — the new formula comes from the charges, and 2+ with 2− gives a one-to-one formula. If you got Ba₂(SO₄)₂: you stopped after criss-crossing — both subscripts divide by 2, and a formula gives the simplest whole-number ratio."
}

[
 {
  "value": "LiN",
  "misconception": "you ignored the charges and paired the atoms one-to-one",
  "feedback": "This answer is wrong — you ignored the charges. N³⁻ needs three Li⁺ partners: Li₃N."
 },
 {
  "value": "LiN₃",
  "misconception": "you crossed the charges in the reverse direction — the 3 belongs on lithium",
  "feedback": "This answer is wrong — you crossed the charges in the reverse direction. Nitrogen's 3 crosses to lithium: Li₃N."
 }
]

{
 "keyExplanation": "Lithium sits in Group 1, so Li⁺; nitrogen sits in Group 15, so its ion is N³⁻. Criss-cross 1 and 3: Li₃N.",
 "anticipatedWrong": [
  {
   "value": "LiN",
   "misconception": "you ignored the charges and paired the atoms one-to-one",
   "feedback": "This answer is wrong — you ignored the charges. N³⁻ needs three Li⁺ partners: Li₃N."
  },
  {
   "value": "LiN₃",
   "misconception": "you crossed the charges in the reverse direction — the 3 belongs on lithium",
   "feedback": "This answer is wrong — you crossed the charges in the reverse direction. Nitrogen's 3 crosses to lithium: Li₃N."
  }
 ]
}

[
 {
  "value": "SnNO₃",
  "misconception": "you ignored the charges and paired one-to-one",
  "feedback": "This answer is wrong — Sn²⁺ needs two NO₃⁻ partners: Sn(NO₃)₂."
 },
 {
  "value": "Sn₂NO₃",
  "misconception": "you crossed the charges in the reverse direction",
  "feedback": "This answer is wrong — the 2 from Sn²⁺ crosses to the nitrate: Sn(NO₃)₂."
 },
 {
  "value": "Sn(NO₃)₃",
  "misconception": "you copied the old compound's subscript pattern instead of re-crossing the charges",
  "feedback": "This answer is wrong — never copy the old compound's subscripts. Re-cross Sn²⁺ with NO₃⁻: Sn(NO₃)₂."
 }
]

{
 "keyExplanation": "Tin takes copper's place and pairs with the nitrate ions. Sn²⁺ and NO₃⁻ criss-cross to Sn(NO₃)₂ — the polyatomic ion stays together as a unit.",
 "anticipatedWrong": [
  {
   "value": "SnNO₃",
   "misconception": "you ignored the charges and paired one-to-one",
   "feedback": "This answer is wrong — Sn²⁺ needs two NO₃⁻ partners: Sn(NO₃)₂."
  },
  {
   "value": "Sn₂NO₃",
   "misconception": "you crossed the charges in the reverse direction",
   "feedback": "This answer is wrong — the 2 from Sn²⁺ crosses to the nitrate: Sn(NO₃)₂."
  },
  {
   "value": "Sn(NO₃)₃",
   "misconception": "you copied the old compound's subscript pattern instead of re-crossing the charges",
   "feedback": "This answer is wrong — never copy the old compound's subscripts. Re-cross Sn²⁺ with NO₃⁻: Sn(NO₃)₂."
  }
 ]
}

[
 {
  "value": "SrCO₃₂",
  "misconception": "you kept both charge numbers instead of letting the matching 2s cancel",
  "feedback": "This answer is wrong — equal charges cancel in the criss-cross: Sr²⁺ and CO₃²⁻ give SrCO₃."
 },
 {
  "value": "Sr₂CO₃",
  "misconception": "you copied the old compound's two-to-one pattern instead of re-crossing the charges",
  "feedback": "This answer is wrong — K₂CO₃ was two-to-one because potassium's ion carries 1. Sr²⁺ matches CO₃²⁻ one-to-one: SrCO₃."
 },
 {
  "value": "Sr(CO₃)₂",
  "misconception": "you crossed the 2s without reducing the matching subscripts",
  "feedback": "This answer is wrong — the matching 2s reduce: SrCO₃."
 }
]

{
 "keyExplanation": "Strontium pairs with the carbonate ion. Sr²⁺ and CO₃²⁻ carry matching 2s, which cancel: SrCO₃ — never copy the old compound's subscript.",
 "anticipatedWrong": [
  {
   "value": "SrCO₃₂",
   "misconception": "you kept both charge numbers instead of letting the matching 2s cancel",
   "feedback": "This answer is wrong — equal charges cancel in the criss-cross: Sr²⁺ and CO₃²⁻ give SrCO₃."
  },
  {
   "value": "Sr₂CO₃",
   "misconception": "you copied the old compound's two-to-one pattern instead of re-crossing the charges",
   "feedback": "This answer is wrong — K₂CO₃ was two-to-one because potassium's ion carries 1. Sr²⁺ matches CO₃²⁻ one-to-one: SrCO₃."
  },
  {
   "value": "Sr(CO₃)₂",
   "misconception": "you crossed the 2s without reducing the matching subscripts",
   "feedback": "This answer is wrong — the matching 2s reduce: SrCO₃."
  }
 ]
}

[
 {
  "value": "KO",
  "misconception": "you ignored the charges and paired the atoms one-to-one",
  "feedback": "This answer is wrong — you ignored the charges and paired the atoms one-to-one. Criss-cross K⁺ and O²⁻: the 2 crosses to potassium, giving K₂O."
 },
 {
  "value": "KO₂",
  "misconception": "you crossed the charge onto the wrong element — the 2 belongs on potassium",
  "feedback": "This answer is wrong — you crossed the charge onto the wrong element. Oxygen's 2 crosses to potassium: K₂O."
 },
 {
  "value": "K₂O₂",
  "misconception": "you kept the 2 from the element formula O₂ as oxygen's subscript — the criss-cross uses ion charges, and the 1 from K⁺ goes unwritten",
  "feedback": "This answer is wrong — the 2 in O₂ counts atoms in the element, not a charge. Criss-cross K⁺ and O²⁻: the 2 crosses to potassium and the unwritten 1 goes to oxygen: K₂O."
 }
]

{
 "keyExplanation": "Potassium sits in Group 1, so its ion is K⁺; oxygen sits in Group 16, so its ion is O²⁻. Criss-cross the charge numbers: K₂O.",
 "anticipatedWrong": [
  {
   "value": "KO",
   "misconception": "you ignored the charges and paired the atoms one-to-one",
   "feedback": "This answer is wrong — you ignored the charges and paired the atoms one-to-one. Criss-cross K⁺ and O²⁻: the 2 crosses to potassium, giving K₂O."
  },
  {
   "value": "KO₂",
   "misconception": "you crossed the charge onto the wrong element — the 2 belongs on potassium",
   "feedback": "This answer is wrong — you crossed the charge onto the wrong element. Oxygen's 2 crosses to potassium: K₂O."
  },
  {
   "value": "K₂O₂",
   "misconception": "you kept the 2 from the element formula O₂ as oxygen's subscript — the criss-cross uses ion charges, and the 1 from K⁺ goes unwritten",
   "feedback": "This answer is wrong — the 2 in O₂ counts atoms in the element, not a charge. Criss-cross K⁺ and O²⁻: the 2 crosses to potassium and the unwritten 1 goes to oxygen: K₂O."
  }
 ]
}

[
 {
  "value": "ZnNO₃",
  "misconception": "you copied the old compound's one-to-one subscript pattern instead of re-crossing the charges",
  "feedback": "This answer is wrong — AgNO₃ was one-to-one because silver's ion carries 1. Zn²⁺ needs two nitrates: Zn(NO₃)₂."
 },
 {
  "value": "Zn₂NO₃",
  "misconception": "you crossed the charges in the reverse direction",
  "feedback": "This answer is wrong — the 2 from Zn²⁺ crosses to the nitrate: Zn(NO₃)₂."
 }
]

{
 "keyExplanation": "Zinc takes silver's place and pairs with nitrate. Zn²⁺ and NO₃⁻ criss-cross to Zn(NO₃)₂ — never copy the old compound's one-to-one pattern.",
 "anticipatedWrong": [
  {
   "value": "ZnNO₃",
   "misconception": "you copied the old compound's one-to-one subscript pattern instead of re-crossing the charges",
   "feedback": "This answer is wrong — AgNO₃ was one-to-one because silver's ion carries 1. Zn²⁺ needs two nitrates: Zn(NO₃)₂."
  },
  {
   "value": "Zn₂NO₃",
   "misconception": "you crossed the charges in the reverse direction",
   "feedback": "This answer is wrong — the 2 from Zn²⁺ crosses to the nitrate: Zn(NO₃)₂."
  }
 ]
}

[
 {
  "value": "FeOH",
  "misconception": "you ignored the charges and paired one-to-one",
  "feedback": "This answer is wrong — Fe³⁺ needs three hydroxide partners: Fe(OH)₃."
 },
 {
  "value": "Fe₃OH",
  "misconception": "you crossed the charges in the reverse direction",
  "feedback": "This answer is wrong — the 3 from Fe³⁺ crosses to the hydroxide: Fe(OH)₃."
 },
 {
  "value": "Fe(OH)₂",
  "misconception": "you used a remembered two, not the supplied charge — the stem gives Fe³⁺",
  "feedback": "This answer is wrong — the stem supplies Fe³⁺, so three hydroxides pair with each iron ion: Fe(OH)₃."
 }
]

{
 "keyExplanation": "Iron pairs with hydroxide. Fe³⁺ and OH⁻ criss-cross to Fe(OH)₃ — the polyatomic ion stays together as a unit.",
 "anticipatedWrong": [
  {
   "value": "FeOH",
   "misconception": "you ignored the charges and paired one-to-one",
   "feedback": "This answer is wrong — Fe³⁺ needs three hydroxide partners: Fe(OH)₃."
  },
  {
   "value": "Fe₃OH",
   "misconception": "you crossed the charges in the reverse direction",
   "feedback": "This answer is wrong — the 3 from Fe³⁺ crosses to the hydroxide: Fe(OH)₃."
  },
  {
   "value": "Fe(OH)₂",
   "misconception": "you used a remembered two, not the supplied charge — the stem gives Fe³⁺",
   "feedback": "This answer is wrong — the stem supplies Fe³⁺, so three hydroxides pair with each iron ion: Fe(OH)₃."
  }
 ]
}

[
 {
  "value": "SrBr",
  "misconception": "you ignored the charges and paired the atoms one-to-one",
  "feedback": "This answer is wrong — you ignored the charges. Sr²⁺ needs two Br⁻ partners: SrBr₂."
 },
 {
  "value": "Sr₂Br",
  "misconception": "you crossed the charges in the reverse direction",
  "feedback": "This answer is wrong — you crossed the charges in the reverse direction. The 2 from Sr²⁺ lands on bromine: SrBr₂."
 }
]

{
 "keyExplanation": "Strontium sits in Group 2, so Sr²⁺; bromine sits in Group 17, so Br⁻. Criss-cross 2 and 1: SrBr₂.",
 "anticipatedWrong": [
  {
   "value": "SrBr",
   "misconception": "you ignored the charges and paired the atoms one-to-one",
   "feedback": "This answer is wrong — you ignored the charges. Sr²⁺ needs two Br⁻ partners: SrBr₂."
  },
  {
   "value": "Sr₂Br",
   "misconception": "you crossed the charges in the reverse direction",
   "feedback": "This answer is wrong — you crossed the charges in the reverse direction. The 2 from Sr²⁺ lands on bromine: SrBr₂."
  }
 ]
}

[
 {
  "value": "MgCl",
  "misconception": "you ignored the charges and paired one-to-one",
  "feedback": "This answer is wrong — Mg²⁺ needs two Cl⁻ partners: MgCl₂."
 },
 {
  "value": "Mg₂Cl",
  "misconception": "you crossed the charges in the reverse direction",
  "feedback": "This answer is wrong — the 2 from Mg²⁺ crosses to chloride: MgCl₂."
 }
]

{
 "keyExplanation": "Magnesium takes nickel's place and pairs with chloride. Mg²⁺ and Cl⁻ criss-cross to MgCl₂.",
 "anticipatedWrong": [
  {
   "value": "MgCl",
   "misconception": "you ignored the charges and paired one-to-one",
   "feedback": "This answer is wrong — Mg²⁺ needs two Cl⁻ partners: MgCl₂."
  },
  {
   "value": "Mg₂Cl",
   "misconception": "you crossed the charges in the reverse direction",
   "feedback": "This answer is wrong — the 2 from Mg²⁺ crosses to chloride: MgCl₂."
  }
 ]
}

[
 {
  "value": "Al₂(OH)₃",
  "misconception": "you copied the old compound's subscript pattern instead of re-crossing the charges",
  "feedback": "This answer is wrong — the 2 and 3 in Al₂(SO₄)₃ belonged to the sulfate pairing. Re-cross Al³⁺ with OH⁻: Al(OH)₃."
 },
 {
  "value": "AlOH",
  "misconception": "you ignored the charges and paired one-to-one",
  "feedback": "This answer is wrong — Al³⁺ needs three hydroxide partners: Al(OH)₃."
 },
 {
  "value": "Al₃OH",
  "misconception": "you crossed the charges in the reverse direction",
  "feedback": "This answer is wrong — the 3 from Al³⁺ crosses to the hydroxide: Al(OH)₃."
 }
]

{
 "keyExplanation": "The Al ions pair with hydroxide. Al³⁺ and OH⁻ criss-cross to Al(OH)₃ — never copy the old compound's subscripts.",
 "anticipatedWrong": [
  {
   "value": "Al₂(OH)₃",
   "misconception": "you copied the old compound's subscript pattern instead of re-crossing the charges",
   "feedback": "This answer is wrong — the 2 and 3 in Al₂(SO₄)₃ belonged to the sulfate pairing. Re-cross Al³⁺ with OH⁻: Al(OH)₃."
  },
  {
   "value": "AlOH",
   "misconception": "you ignored the charges and paired one-to-one",
   "feedback": "This answer is wrong — Al³⁺ needs three hydroxide partners: Al(OH)₃."
  },
  {
   "value": "Al₃OH",
   "misconception": "you crossed the charges in the reverse direction",
   "feedback": "This answer is wrong — the 3 from Al³⁺ crosses to the hydroxide: Al(OH)₃."
  }
 ]
}

[
 {
  "value": "RbCl₂",
  "misconception": "you copied the 2 from the element formula Cl₂ into the product — the criss-cross uses ion charges, not the diatomic 2",
  "feedback": "This answer is wrong — the 2 in Cl₂ counts atoms in the element, not the ion's charge. Rb⁺ and Cl⁻ pair one-to-one: RbCl."
 },
 {
  "value": "Rb₂Cl",
  "misconception": "you doubled the metal with no charge to justify it",
  "feedback": "This answer is wrong — both ions carry a 1, so the atoms pair one-to-one: RbCl."
 }
]

{
 "keyExplanation": "Rubidium sits in Group 1, so Rb⁺; chlorine sits in Group 17, so Cl⁻. Criss-cross 1 and 1 — both go unwritten: RbCl.",
 "anticipatedWrong": [
  {
   "value": "RbCl₂",
   "misconception": "you copied the 2 from the element formula Cl₂ into the product — the criss-cross uses ion charges, not the diatomic 2",
   "feedback": "This answer is wrong — the 2 in Cl₂ counts atoms in the element, not the ion's charge. Rb⁺ and Cl⁻ pair one-to-one: RbCl."
  },
  {
   "value": "Rb₂Cl",
   "misconception": "you doubled the metal with no charge to justify it",
   "feedback": "This answer is wrong — both ions carry a 1, so the atoms pair one-to-one: RbCl."
  }
 ]
}

[
 {
  "value": "AlCl₂",
  "misconception": "you copied the old compound's subscript instead of re-crossing the charges",
  "feedback": "This answer is wrong — the 2 belonged to copper's compound. Re-cross Al³⁺ with Cl⁻: AlCl₃."
 },
 {
  "value": "AlCl",
  "misconception": "you ignored the charges and paired one-to-one",
  "feedback": "This answer is wrong — Al³⁺ needs three Cl⁻ partners: AlCl₃."
 },
 {
  "value": "Al₃Cl",
  "misconception": "you crossed the charges in the reverse direction",
  "feedback": "This answer is wrong — the 3 from Al³⁺ crosses to chloride: AlCl₃."
 }
]

{
 "keyExplanation": "Aluminum takes copper's place and pairs with chloride. Al³⁺ and Cl⁻ criss-cross to AlCl₃ — never copy the old compound's subscript.",
 "anticipatedWrong": [
  {
   "value": "AlCl₂",
   "misconception": "you copied the old compound's subscript instead of re-crossing the charges",
   "feedback": "This answer is wrong — the 2 belonged to copper's compound. Re-cross Al³⁺ with Cl⁻: AlCl₃."
  },
  {
   "value": "AlCl",
   "misconception": "you ignored the charges and paired one-to-one",
   "feedback": "This answer is wrong — Al³⁺ needs three Cl⁻ partners: AlCl₃."
  },
  {
   "value": "Al₃Cl",
   "misconception": "you crossed the charges in the reverse direction",
   "feedback": "This answer is wrong — the 3 from Al³⁺ crosses to chloride: AlCl₃."
  }
 ]
}

[
 {
  "value": "BaOH",
  "misconception": "you ignored the charges and paired one-to-one",
  "feedback": "This answer is wrong — Ba²⁺ needs two hydroxide partners: Ba(OH)₂."
 },
 {
  "value": "Ba₂OH",
  "misconception": "you crossed the charges in the reverse direction",
  "feedback": "This answer is wrong — the 2 from Ba²⁺ crosses to the hydroxide: Ba(OH)₂."
 },
 {
  "value": "BaCl(OH)",
  "misconception": "you let the barium keep one old partner — in the trade, each positive ion pairs fully with the OTHER compound's negative ion",
  "feedback": "This answer is wrong — barium trades completely to the hydroxide: Ba(OH)₂."
 }
]

{
 "keyExplanation": "Barium pairs with hydroxide. Ba²⁺ and OH⁻ criss-cross to Ba(OH)₂ — the polyatomic ion stays together as a unit.",
 "anticipatedWrong": [
  {
   "value": "BaOH",
   "misconception": "you ignored the charges and paired one-to-one",
   "feedback": "This answer is wrong — Ba²⁺ needs two hydroxide partners: Ba(OH)₂."
  },
  {
   "value": "Ba₂OH",
   "misconception": "you crossed the charges in the reverse direction",
   "feedback": "This answer is wrong — the 2 from Ba²⁺ crosses to the hydroxide: Ba(OH)₂."
  },
  {
   "value": "BaCl(OH)",
   "misconception": "you let the barium keep one old partner — in the trade, each positive ion pairs fully with the OTHER compound's negative ion",
   "feedback": "This answer is wrong — barium trades completely to the hydroxide: Ba(OH)₂."
  }
 ]
}