Phosphoric acid has the formula H₃PO₄. State what is meant by the term relative formula mass and describe the steps used to find the relative formula mass of phosphoric acid. The relative atomic masses (Aᵣ) you will need are: H = 1, O = 16, P = 31.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.4 Relative formula mass · State and Describe · 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 (in the ratio shown by the formula).

To find the relative formula mass of H₃PO₄:
1. Count the atoms: 3 H, 1 P, 4 O.
2. Multiply each element’s relative atomic mass by its number of atoms:
3 × 1 = 3 (H)
1 × 31 = 31 (P)
4 × 16 = 64 (O)
3. Add the values: 3 + 31 + 64 = 98.

The relative formula mass of phosphoric acid is 98.

Examiner tips

  • Use the exact definition of relative formula mass. Show the step‑by‑step calculation with each multiplication. State the final value clearly.
  • Use the given atomic masses; don’t round intermediate steps.

Common mistakes

  • Using the wrong atomic mass for an element. Omitting the multiplication step for one element. Failing to add all three contributions together.

Mark scheme (4 marks)

  1. Relative formula mass is the sum of the relative atomic masses of all the atoms in a formula (in the ratio shown by the formula).
  2. Identify the number of each type of atom in H₃PO₄: 3 hydrogen atoms, 1 phosphorus atom, and 4 oxygen atoms.
  3. Multiply the relative atomic mass of each element by the number of its atoms: (3 × 1) for H, (1 × 31) for P, and (4 × 16) for O.
  4. Add the values together to give the relative formula mass: 3 + 31 + 64 = 98.

Key terms in this question

relative formula mass · relative atomic mass · formula

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