# 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

## 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

- [relative formula mass](https://www.gradenine.co.uk/glossary/relative-formula-mass)
- [relative atomic mass](https://www.gradenine.co.uk/glossary/relative-atomic-mass)
- [formula](https://www.gradenine.co.uk/glossary/formula)

## Related

- [Revision notes for Pearson Edexcel International GCSE Chemistry (4CH1)](https://www.gradenine.co.uk/learn)
- [How to answer "State and Describe" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/phosphoric-acid-has-the-formula-h-po-de01256c) · Published by Druglandscape Ltd.