GCSE & IGCSE chemistry practice

Atom economy questions and answers

Atom economy measures how much of the reactants ends up in the useful product: % atom economy $= \dfrac{M_r \text{ of desired product}}{\text{total } M_r \text{ of all reactants}} \times 100$ (use the numbers from the balanced equation). A high atom economy means less waste.

8 questions · Foundation tier · Every answer is checked as you type it, with a full worked solution. Updated 27 Oct 2026.

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Where atom economy comes up

GCSE (AQA)	GCSE Chemistry only (not Combined Science), Foundation and Higher: calculate the percentage atom economy from the balanced equation. Higher: explain why a particular reaction pathway is chosen.
Cambridge IGCSE (0620)	Atom economy isn't in the 2026 to 2028 syllabus.

The method

Use the balanced equation, including the numbers in front of formulae.
Add up the Mr of all the reactants (or all the products: it's the same).
Mr of the desired product (× its number in the equation) ÷ total × 100.
Only one product, or all products useful: 100%.

Atom economy questions

Try each one before looking at the hint. If you're stuck, the AI tutor can talk you through it step by step.

Warm-up

Question 1Warm-upFoundation

$\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$ . Calculate the atom economy for making calcium oxide. ( $M_r$ : CaCO₃ = 100, CaO = 56)

Question 2Warm-upFoundation

$\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3$ . What is the atom economy for making ammonia?

Exam-style

Question 3Exam-styleFoundation

$\text{C}_2\text{H}_5\text{OH} \rightarrow \text{C}_2\text{H}_4 + \text{H}_2\text{O}$ . Calculate the atom economy for making ethene to 1 decimal place. ( $M_r$ : C₂H₅OH = 46, C₂H₄ = 28)

Question 4Exam-styleFoundation

$\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2$ . Calculate the atom economy for making iron to 1 decimal place. ( $M_r$ : Fe₂O₃ = 160, CO = 28; $A_r$ Fe = 56)

Question 5Exam-styleFoundation

$\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$ . Calculate the atom economy for making hydrogen to 2 significant figures. ( $A_r$ : Zn = 65, H = 1, Cl = 35.5)

Question 6Exam-styleFoundation

$\text{CuO} + \text{H}_2\text{SO}_4 \rightarrow \text{CuSO}_4 + \text{H}_2\text{O}$ . Calculate the atom economy for making copper sulfate to 1 decimal place. ( $M_r$ : CuO = 79.5, H₂SO₄ = 98, CuSO₄ = 159.5)

Challenge

Question 7ChallengeFoundation

Two routes make the same product. Route A has an atom economy of 45%; route B has 92%. Give one reason a company might choose route B.

Question 8ChallengeFoundation

Making ethanol: $\text{C}_2\text{H}_4 + \text{H}_2\text{O} \rightarrow \text{C}_2\text{H}_5\text{OH}$ . What is the atom economy?

Common mistakes

Using the Mr of the product without its balancing number.
Dividing by the products' total instead of the reactants' total (they're equal, but students often miss a reactant).
Confusing atom economy with percentage yield.

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Sources

The questions are our own. The “where it comes up” notes are from each board's published specification, checked on 27 October 2026:

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