Nitrogen forms a compound with hydrogen called ammonia, NH₃. Explain how the relative formula mass of ammonia is determined, and state why the relative formula mass has no units.

Cambridge International IGCSE Chemistry (0620) — 3.2 Relative masses of atoms and molecules · 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 relative formula mass of NH₃ is found by adding the relative atomic masses of the atoms in the formula.
1. N has a relative atomic mass of 14.
2. H has a relative atomic mass of 1, and there are three H atoms.
3. 14 + 3×1 = 17.
Thus the relative formula mass of ammonia is 17.
The relative formula mass has no units because it is a ratio of the mass of the compound to the mass of a reference atom (carbon‑12 or 1/12 the mass of a carbon‑12 atom). Both masses are expressed in the same units, so the units cancel out.

Examiner tips

  • Show the calculation step‑by‑step, including the factor of 3 for hydrogen.
  • State the atomic masses used (N = 14, H = 1).
  • Explain that the result is unitless because it is a ratio of masses.

Common mistakes

  • Using wrong atomic masses (e.g., N = 15).
  • Omitting the factor of 3 for hydrogen.
  • Claiming the relative formula mass has units such as amu or g/mol.

Mark scheme (4 marks)

  1. Relative formula mass is found by adding together the relative atomic masses of all atoms in the formula
  2. Correct use of Ar values: N = 14, H = 1 (×3), giving 14 + 3 = 17
  3. Relative formula mass is a ratio / comparison to carbon-12 (or to 1/12 the mass of a carbon-12 atom)
  4. Because both masses being compared are in the same units, the units cancel

Key terms in this question

relative formula mass

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