# Nitrogen gas reacts with hydrogen gas to form ammonia gas. The equation for the reaction is: N₂ + 3H₂ → 2NH₃. Use Avogadro's law to explain what the equation tells us about the volumes of gases involved, and state one condition that must be met for this to be true.

> AQA GCSE Chemistry (8462) — 4.3.5 Use of amount of substance in relation to volumes of gases (Chem only) · Explain · 4 marks

## Mark scheme (4 marks)

1. Avogadro's law states that equal amounts in moles of gases occupy the same volume under the same conditions of temperature and pressure
2. The ratio of volumes of N₂ : H₂ : NH₃ is 1 : 3 : 2 (matching the molar ratio from the equation)
3. The total volume of reactant gases (4 volumes) is greater than the total volume of product gas (2 volumes), so the volume decreases
4. Temperature and pressure must be the same (constant / identical) for all gases compared

## Key terms

- [Avogadro's law](https://www.gradenine.co.uk/glossary/avogadro-s-law)
- [volume](https://www.gradenine.co.uk/glossary/volume)

## Related

- [Revision notes for AQA GCSE Chemistry (8462)](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-gas-reacts-with-hydrogen-gas-b4b97239) · Published by Druglandscape Ltd.