# Explain, using the kinetic model of an ideal gas, why two different gases that occupy the same volume at the same temperature and pressure contain the same number of molecules.

> IB DP Physics Higher Level (2023 syllabus) — B.3 Gas laws · Explain · 4 marks

> This relationship is known as Avogadro's law and applies to all ideal gases regardless of the nature of the gas molecules.

## Mark scheme (4 marks)

1. Pressure depends on the rate of change of momentum of molecules colliding with the container walls.
2. At the same temperature, molecules of both gases have the same mean kinetic energy.
3. For the same pressure and volume, the total force (or total rate of momentum transfer) on the walls must be identical for both gases.
4. Because the mean kinetic energy per molecule is the same, the number of molecules must be equal in both gases.

## Key terms

- [kinetic model](https://www.gradenine.co.uk/glossary/kinetic-model)
- [pressure](https://www.gradenine.co.uk/glossary/pressure)

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-using-the-kinetic-model-of-3427decd) · Published by Druglandscape Ltd.