Describe what is meant by the term empirical formula and explain how the empirical formula of a compound may differ from its molecular formula. Use the compound ethane, C₂H₆, as an example in your answer.

Cambridge International IGCSE Chemistry (0620) — 3.1 Formulae · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

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

The molecular formula shows the actual number of atoms of each element in one molecule.

For ethane the empirical formula is CH₃.

The molecular formula C₂H₆ is a whole‑number multiple (2×) of the empirical formula, so the two formulas are different.

Examiner tips

  • Use the exact terms "empirical formula" and "molecular formula". Show the ratio for ethane (CH₃) and state that C₂H₆ is twice this. Keep the answer concise to fit the 4 marks.

Common mistakes

  • Confusing empirical formula with molecular formula. Giving the empirical formula as C₂H₆ instead of CH₃. Not recognising that the molecular formula is a multiple of the empirical formula.

Mark scheme (4 marks)

  1. Empirical formula is the simplest whole-number ratio of atoms of each element in a compound
  2. The molecular formula shows the actual number of atoms of each element in one molecule
  3. The empirical formula of ethane is CH₃
  4. The molecular formula (C₂H₆) is a whole-number multiple of the empirical formula (CH₃), so they are different / the molecular formula is twice the empirical formula

Key terms in this question

empirical formula · molecular formula

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