Ammonium sulfate, (NH₄)₂SO₄, is used as a fertiliser. State what is meant by the term relative formula mass and use the formula of ammonium sulfate to show how its relative formula mass is determined. (Relative atomic masses: H = 1, N = 14, O = 16, S = 32)

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

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

Ammonium sulfate has the formula (NH₄)₂SO₄. It is widely used in agriculture as a nitrogen-containing fertiliser.

Model answer (4 marks)

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

For ammonium sulfate, (NH₄)₂SO₄, the atoms are:
2 N, 8 H, 1 S, 4 O.

Using the relative atomic masses:
(2 × 14) + (8 × 1) + (1 × 32) + (4 × 16) = 28 + 8 + 32 + 64 = 132.

The relative formula mass of ammonium sulfate is 132 (no units).

Examiner tips

  • Define the term first, then list all atoms with correct counts, substitute the atomic masses, and finish with the numerical result.
  • Show the calculation step‑by‑step to demonstrate understanding of the formula expansion.

Common mistakes

  • Failing to expand the brackets and miscounting atoms; e.g., writing 4 N instead of 2 N.
  • Using incorrect atomic masses or forgetting to multiply by the number of atoms before adding.

Mark scheme (4 marks)

  1. Relative formula mass is the sum of the relative atomic masses of all the atoms in the formula (of the compound)
  2. Correct identification of all atoms present: 2N, 8H, 1S, 4O (i.e. correctly expanding the brackets in (NH₄)₂SO₄)
  3. Correct substitution of relative atomic masses: (2 × 14) + (8 × 1) + (1 × 32) + (4 × 16) = 28 + 8 + 32 + 64
  4. Correct final answer of 132 (no units)

Key terms in this question

relative formula mass · relative atomic mass · formula

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