Potassium manganate(VII) has the formula KMnO₄. State the number of atoms of each element present in one formula unit of potassium manganate(VII). Then explain what the term empirical formula means and state whether KMnO₄ is also the empirical formula of potassium manganate(VII), giving a reason for your answer.

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

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

Model answer (4 marks)

One formula unit of KMnO₄ contains 1 potassium atom, 1 manganese atom and 4 oxygen atoms.

The empirical formula is the simplest whole‑number ratio of the atoms of each element in a compound.

KMnO₄ is the empirical formula of potassium manganate(VII) because the ratio 1:1:4 cannot be simplified further to give a smaller whole‑number ratio.

Examiner tips

  • State the atom counts clearly; use the correct symbols. Explain the definition of empirical formula. Show that 1:1:4 is already the simplest ratio. Use the exact wording from the mark scheme.

Common mistakes

  • Writing the wrong atom counts (e.g. 2 K or 5 O). Confusing empirical formula with molecular formula. Claiming the formula can be simplified when it cannot.

Mark scheme (4 marks)

  1. One formula unit of KMnO₄ contains 1 potassium atom, 1 manganese atom and 4 oxygen atoms
  2. Empirical formula is the simplest whole number ratio of atoms of each element in a compound
  3. KMnO₄ is the empirical formula of potassium manganate(VII)
  4. Because the ratio of atoms (1 : 1 : 4) cannot be simplified / divided further to give smaller whole numbers

Key terms in this question

empirical formula · formula unit

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