In the thermal decomposition of limestone, CaCO3 → CaO + CO2, the useful product is calcium oxide (CaO). Calculate the atom economy for making calcium oxide. (Mr: CaCO3 = 100, CaO = 56)
[2 marks]
Total for this question: 2
All calculations
Atom economy
Quantitative chemistry
AQA 8462
ORIGINAL
atom economy = (mass of desired products / total mass of reactants) x 100; use balancing numbers x Mr
It's about the balanced equation, not the experiment — desired product over the reactant mass.
Board and spec code confirmed against AQA 8462 · registry checked 2026-07-17How this checking works
Work it, then mark it
Do each calculation on paper first, then reveal the mark scheme and tick the marks you actually earned — the same way you should mark past papers.
Q1
Do the calculation on paper first — then mark it.
Where the marks get lost
- Confusing atom economy with percentage yield — atom economy uses the balanced equation, not measured masses.
- Only counting one reactant when several react (add all reactant Mr values).
- Forgetting to multiply by 100 for a percentage.
Exam tip: Atom economy is fixed by the equation, so no experiment is involved. A reaction with only one product has 100% atom economy.
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