# 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

## 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

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

## Related

- [Revision notes for Cambridge International IGCSE Chemistry (0620)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/nitrogen-forms-a-compound-with-hydrogen-b72d16f3) · Published by Druglandscape Ltd.