GCSE & IGCSE chemistry practice

Reacting masses questions and answers

Reacting-mass questions use a balanced equation to find how much product you get (or reactant you need). The method: find the moles of what you know, use the equation's ratio to get the moles of what you want, then convert back to mass. Relative atomic masses: H = 1, C = 12, N = 14, O = 16, Na = 23, Mg = 24, Al = 27, S = 32, Cl = 35.5, K = 39, Ca = 40, Fe = 56, Cu = 63.5.

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

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Where reacting masses comes up

GCSE (AQA)	Chemistry and Combined Science, Higher only: calculate the masses of reactants and products from balanced equations using moles.
Cambridge IGCSE (0620)	Core: reacting masses in simple proportions (without the mole concept). Supplement: stoichiometric reacting masses using moles.

The method

Moles of the substance you know = mass ÷ Mr.
Use the balanced equation's ratio to find the moles of the substance you want.
Mass = moles × Mr.
Simple proportion also works when the ratio is 1 : 1 (e.g. 40 g gives 56 g, so 20 g gives 28 g).

Reacting masses 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-upHigher

$\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$ . What mass of calcium oxide forms from 50 g of calcium carbonate? (Ca = 40, C = 12, O = 16)

Question 2Warm-upHigher

$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$ . What mass of magnesium oxide forms when 12 g of magnesium burns? (Mg = 24, O = 16)

Exam-style

Question 3Exam-styleHigher

$2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$ . What mass of water forms from 4 g of hydrogen? (H = 1, O = 16)

Question 4Exam-styleHigher

$\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2$ . What mass of iron can be made from 320 g of iron(III) oxide? (Fe = 56, O = 16)

Question 5Exam-styleHigher

$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$ . What mass of carbon dioxide is made when 8 g of methane burns? (C = 12, H = 1, O = 16)

Question 6Exam-styleHigher

$\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2$ . What mass of hydrochloric acid reacts with 6 g of magnesium? (Mg = 24, H = 1, Cl = 35.5)

Challenge

Question 7ChallengeHigher

$2\text{Al} + \text{Fe}_2\text{O}_3 \rightarrow \text{Al}_2\text{O}_3 + 2\text{Fe}$ . What mass of aluminium is needed to make 112 g of iron? (Al = 27, Fe = 56)

Question 8ChallengeHigher

$\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3$ . What mass of ammonia can be made from 6 g of hydrogen? (N = 14, H = 1)

Common mistakes

Ignoring the numbers in front of formulae in the equation.
Using the mass ratio as if it were the mole ratio.
Working out the Mr of 2H₂O as 18 instead of using 2 moles of 18.

Stuck on one of these?

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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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