Sodium sulfate has the formula Na₂SO₄. State what is meant by the term relative formula mass and calculate the relative formula mass of sodium sulfate. (Relative atomic masses: Na = 23, S = 32, 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 the atoms in a formula.

2 × Na = 2 × 23 = 46
1 × S = 1 × 32 = 32
4 × O = 4 × 16 = 64

46 + 32 + 64 = 142

Examiner tips

  • Use the exact definition of relative formula mass. Show the calculation for each element. Add the partial sums before giving the final total. Use the given relative atomic masses.
  • common_mistakes
  • :
  • Forgetting to multiply the atomic mass of oxygen by 4. Using the wrong atomic mass for sodium or sulfur. Omitting the definition of relative formula mass.

Mark scheme (4 marks)

  1. Relative formula mass is the sum of the relative atomic masses of all the atoms in a formula
  2. Correct identification/working: 2 × Na = 2 × 23 = 46
  3. Correct identification/working: 1 × S = 1 × 32 = 32
  4. Correct final answer of 142

Key terms in this question

relative formula mass · relative atomic mass

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