Calcium carbonate has the formula CaCO₃. Define the term relative formula mass and describe the steps a student would take to find the relative formula mass of calcium carbonate, using the periodic table. (Atomic masses: Ca = 40, C = 12, O = 16)

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

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

Model answer (4 marks)

Define: Relative formula mass is the sum of the relative atomic masses of all atoms in the formula.

Steps to find the relative formula mass of CaCO₃:
1. Identify the number of each type of atom: 1 Ca, 1 C, 3 O.
2. Multiply each element’s relative atomic mass by its number of atoms:
Ca: 40 × 1 = 40
C: 12 × 1 = 12
O: 16 × 3 = 48
3. Add the values: 40 + 12 + 48 = 100.

The relative formula mass of CaCO₃ is 100.

Examiner tips

  • Use the exact definition first; examiners look for the phrase ‘sum of the relative atomic masses’.
  • Show the multiplication for each element and the final addition to demonstrate the calculation.

Mark scheme (4 marks)

  1. Relative formula mass is the sum of the relative atomic masses of all atoms in the formula
  2. Identify the number of each type of atom: one Ca, one C, and three O atoms
  3. Multiply each element's relative atomic mass by its number of atoms: 40×1, 12×1, 16×3
  4. Add all the values together to give a relative formula mass of 100

Key terms in this question

relative formula mass · formula

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