Aluminium sulfate has the formula Al₂(SO₄)₃. State the number of atoms of each element present in one formula unit of aluminium sulfate, and use this information to explain what the formula tells us about the empirical formula of aluminium sulfate.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.5 Chemical formulae, equations and calculations · State · 4 marks · View as Markdown

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

Aluminium sulfate, Al₂(SO₄)₃, is used in water treatment and in the paper industry.

Model answer (4 marks)

Aluminium sulfate contains 2 Al atoms, 3 S atoms and 12 O atoms in one formula unit.
The formula Al₂(SO₄)₃ shows that the simplest whole‑number ratio of the elements is 2:3:12, which cannot be reduced further. Therefore Al₂(SO₄)₃ is already the empirical formula.

Examiner tips

  • State the exact numbers of each element first; then explain the ratio cannot be simplified.
  • Use the word ‘empirical’ and mention the lack of a common factor >1.
  • Keep the answer concise – no extra words or explanations.

Mark scheme (4 marks)

  1. 2 aluminium atoms identified
  2. 3 sulfur atoms identified
  3. 12 oxygen atoms identified
  4. Al₂(SO₄)₃ is already the empirical formula because the ratio 2:3:12 cannot be simplified to smaller whole numbers / the numbers 2, 3 and 12 share no common factor greater than 1 that reduces all three simultaneously

Key terms in this question

empirical formula · formula unit

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