GCSE & IGCSE chemistry practice

Relative formula mass questions and answers

The relative formula mass ( $M_r$ ) of a compound is the sum of the relative atomic masses ( $A_r$ ) of all the atoms in its formula. Multiply each $A_r$ by how many of that atom there are; a number after a bracket multiplies everything inside it. 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.

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Where relative formula mass comes up

GCSE (AQA)	Chemistry and Combined Science, Foundation and Higher: calculate relative formula mass, and know that in a balanced equation the total Mr of the reactants equals the total Mr of the products.
Cambridge IGCSE (0620)	Core: define relative molecular mass, Mr, as the sum of the relative atomic masses (relative formula mass for ionic compounds).

The method

Count the atoms of each element in the formula.
Multiply each count by that element's Ar.
A number after a bracket multiplies every atom inside: Ca(OH)₂ has 2 O and 2 H.
Add everything up. Mr has no units.

Relative formula mass 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

Calculate the $M_r$ of water, $\text{H}_2\text{O}$ . (H = 1, O = 16)

Question 2Warm-upFoundation

Calculate the $M_r$ of carbon dioxide, $\text{CO}_2$ . (C = 12, O = 16)

Exam-style

Question 3Exam-styleFoundation

Calculate the $M_r$ of sodium chloride, $\text{NaCl}$ . (Na = 23, Cl = 35.5)

Question 4Exam-styleFoundation

Calculate the $M_r$ of sulfuric acid, $\text{H}_2\text{SO}_4$ . (H = 1, S = 32, O = 16)

Question 5Exam-styleFoundation

Calculate the $M_r$ of calcium hydroxide, $\text{Ca(OH)}_2$ . (Ca = 40, O = 16, H = 1)

Question 6Exam-styleFoundation

Calculate the $M_r$ of magnesium nitrate, $\text{Mg(NO}_3)_2$ . (Mg = 24, N = 14, O = 16)

Challenge

Question 7ChallengeFoundation

Calculate the $M_r$ of aluminium sulfate, $\text{Al}_2(\text{SO}_4)_3$ . (Al = 27, S = 32, O = 16)

Question 8ChallengeFoundation

In the reaction $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$ , show that mass is conserved by calculating the total $M_r$ of the products. (Ca = 40, C = 12, O = 16)

Common mistakes

Only multiplying the last atom in a bracket.
Adding the small numbers instead of using them as multipliers.
Using the atomic number instead of the relative atomic mass.

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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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