GCSE & IGCSE chemistry practice

Percentage by mass questions and answers

The percentage by mass of an element in a compound is: $\dfrac{A_r \times \text{number of atoms}}{M_r} \times 100$ . It tells you how much of the compound's mass comes from that element. 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 · Foundation tier · Every answer is checked as you type it, with a full worked solution. Updated 27 Oct 2026.

Start the questions Percentage by mass calculator

Where percentage by mass comes up

GCSE (AQA)	Chemistry and Combined Science, Foundation and Higher: calculate the percentage by mass in a compound given the relative formula mass and relative atomic masses.
Cambridge IGCSE (0620)	Supplement (Extended): calculate percentage composition by mass from data.

The method

Work out the Mr of the compound.
Work out the total mass of the element: Ar × number of atoms.
% by mass = element's mass ÷ Mr × 100.
Mass of element in a sample = % ÷ 100 × mass of sample.

Percentage by mass questions

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Warm-up

Question 1Warm-upFoundation

What is the percentage by mass of carbon in carbon dioxide, $\text{CO}_2$ ? Give your answer to 1 decimal place. (C = 12, O = 16)

Question 2Warm-upFoundation

What is the percentage by mass of oxygen in water, $\text{H}_2\text{O}$ ? Give your answer to 1 decimal place. (H = 1, O = 16)

Exam-style

Question 3Exam-styleFoundation

What is the percentage by mass of calcium in calcium carbonate, $\text{CaCO}_3$ ? (Ca = 40, C = 12, O = 16)

Question 4Exam-styleFoundation

What is the percentage by mass of nitrogen in ammonium nitrate, $\text{NH}_4\text{NO}_3$ ? (N = 14, H = 1, O = 16)

Question 5Exam-styleFoundation

What is the percentage by mass of iron in iron(III) oxide, $\text{Fe}_2\text{O}_3$ ? (Fe = 56, O = 16)

Question 6Exam-styleFoundation

What mass of iron can be extracted from 200 kg of iron(III) oxide, which is 70% iron by mass?

Challenge

Question 7ChallengeFoundation

What is the percentage by mass of water in hydrated copper sulfate, $\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$ ? Give your answer to 1 decimal place. (Cu = 63.5, S = 32, O = 16, H = 1)

Question 8ChallengeFoundation

Which fertiliser has the higher percentage of nitrogen: urea, $\text{CO(NH}_2)_2$ ( $M_r = 60$ ), or ammonium sulfate, $(\text{NH}_4)_2\text{SO}_4$ ( $M_r = 132$ )? Give the higher percentage to 1 decimal place.

Common mistakes

Forgetting to multiply by the number of atoms.
Dividing by the wrong Mr.
Forgetting the × 100.

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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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