Ammonia has the molecular formula NH₃. State the meaning of this molecular formula and explain why the empirical formula and the molecular formula of ammonia are the same.

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

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

Model answer (4 marks)

The molecular formula shows the actual number of atoms of each element in one molecule of the compound
One molecule of ammonia contains one nitrogen atom and three hydrogen atoms
The empirical formula shows the simplest whole‑number ratio of atoms of each element in the compound
The ratio 1:3 (N:H) in NH₃ cannot be simplified further / is already the simplest whole‑number ratio, so the empirical formula is also NH₃

Examiner tips

  • Use the exact wording ‘molecular formula’ and ‘empirical formula’
  • Show the ratio 1:3 and state it cannot be simplified
  • Explain that the empirical formula is the simplest ratio

Common mistakes

  • Writing ‘molecular formula shows the ratio of atoms’ instead of the actual number
  • Confusing the empirical formula with the molecular formula and giving a different ratio
  • Omitting the explanation that 1:3 cannot be simplified

Mark scheme (4 marks)

  1. The molecular formula shows the actual number of atoms of each element in one molecule of the compound
  2. One molecule of ammonia contains one nitrogen atom and three hydrogen atoms
  3. The empirical formula shows the simplest whole-number ratio of atoms of each element in the compound
  4. The ratio 1:3 (N:H) in NH₃ cannot be simplified further / is already the simplest whole-number ratio, so the empirical formula is also NH₃

Key terms in this question

molecular formula · empirical formula

Related

More Formulae questions

▶ Try answering this question with AI marking (free) →