Calcium carbonate has the formula CaCO₃. Explain how to calculate the relative formula mass of calcium carbonate, and state its value. (Relative atomic masses: Ca = 40, C = 12, O = 16)

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.4 Relative formula mass · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The relative formula mass of CaCO₃ is found by adding the relative atomic masses of all atoms in the formula.
1. Identify the atoms: 1 Ca, 1 C, 3 O.
2. Calculate: 40 + 12 + (3 × 16) = 40 + 12 + 48 = 100.
The relative formula mass is 100.

Examiner tips

Use the command word ‘Explain’ to describe the process, not just give the number.
Show the calculation step‑by‑step with the correct symbols (Ca, C, O).
State the final value in the last sentence.
Use UK spelling (e.g. ‘formula mass’).

Common mistakes

Writing the formula mass as 99 or 101 due to mis‑adding the oxygen contribution.
Omitting the number of oxygen atoms (writing 40 + 12 + 16).
Using the wrong atomic mass for Ca or O (e.g. 41 or 15).

Mark scheme (4 marks)

Identifies that the relative formula mass is found by adding together the relative atomic masses of all atoms in the formula
Correctly identifies the number of each type of atom: one Ca, one C, and three O atoms
Shows correct working: 40 + 12 + (3 × 16)
States the correct relative formula mass of 100

Key terms in this question

relative formula mass · relative atomic mass