Copper(II) sulfate has the formula CuSO₄. State what is meant by the term relative formula mass and describe the steps used to work out the relative formula mass of copper(II) sulfate. Use the relative atomic masses: Cu = 64, S = 32, O = 16.

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

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

Model answer (4 marks)

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

To work out the relative formula mass of CuSO₄:
1. Identify the atoms in the formula: 1 Cu, 1 S, 4 O.
2. Multiply each atom’s relative atomic mass by its number:
1 × 64 = 64
1 × 32 = 32
4 × 16 = 64
3. Add the values: 64 + 32 + 64 = 160.

The relative formula mass of copper(II) sulfate is 160.

Examiner tips

  • Use the exact definition of relative formula mass; include the word ‘sum’.
  • Show the multiplication for each element and the final addition.
  • State the final value clearly.

Common mistakes

  • Writing the wrong number of oxygen atoms (e.g., 3 O instead of 4).
  • Omitting the multiplication step and just adding the atomic masses.

Mark scheme (4 marks)

  1. Relative formula mass is the sum of the relative atomic masses of all the atoms in a formula
  2. Identify the correct number of each atom in CuSO₄: 1 Cu, 1 S, 4 O
  3. Multiply each relative atomic mass by the number of that atom present: 1 × 64, 1 × 32, 4 × 16
  4. Adds all values correctly to give a relative formula mass of 160

Key terms in this question

relative formula mass · relative atomic mass

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