Atom economy
atom economy = (Mr of desired product / sum of Mr of all reactants) x 100
It's about the balanced equation, not the experiment — desired product over the reactant mass.
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.
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)
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.
Calculations are the most trainable marks in chemistry
They come up every paper and reward a clean method. Send me one you keep dropping marks on — your first lesson is free.