Iron(III) oxide has the formula Fe₂O₃. State what is meant by the term relative formula mass and use the formula of iron(III) oxide to show how the relative formula mass is determined. (Relative atomic masses: Fe = 56, O = 16)

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.4 Relative formula mass · State · 4 marks · View as Markdown

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

Model answer (4 marks)

Relative formula mass is the sum of the relative atomic masses of all atoms in the formula of the compound.

Fe₂O₃ contains 2 Fe atoms and 3 O atoms.

2 × 56 = 112 (Fe) and 3 × 16 = 48 (O).

112 + 48 = 160.

The relative formula mass of Fe₂O₃ is 160.

Examiner tips

  • Define the term first, then list the atoms and their counts, show the multiplication, and give the final sum.
  • Use the exact wording ‘relative formula mass’ and ‘relative atomic masses’ to match the mark scheme.

Common mistakes

  • Writing the wrong number of atoms (e.g., 3 Fe instead of 2), Using atomic weights instead of relative atomic masses, or Omitting the final sum of 160.

Mark scheme (4 marks)

  1. Relative formula mass is the sum of the relative atomic masses of all atoms in the formula (of the compound)
  2. Correctly identifies the number of each type of atom: 2 Fe atoms and 3 O atoms
  3. Correct multiplication: 2 × 56 = 112 for iron, and 3 × 16 = 48 for oxygen
  4. Correct final answer of 160

Key terms in this question

relative formula mass · relative atomic mass · formula

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