# Explain why the molar volume of a real gas at high pressure is greater than the value predicted by the ideal gas equation.

> IB DP Chemistry Higher Level (2023 syllabus) — S1.5 Ideal gases · Explain · 4 marks

## Mark scheme (4 marks)

1. The ideal gas model assumes that gas molecules/particles occupy negligible/zero volume compared to the total volume of the container.
2. At high pressure, gas molecules are forced close together so the volume occupied by the molecules themselves becomes a significant fraction of the total volume.
3. The actual/real volume available for movement is therefore greater than predicted by PV = nRT, because the molecules themselves take up space.
4. At high pressure the repulsive (short-range) intermolecular forces also become significant, pushing molecules apart and further increasing the volume beyond the ideal prediction.

## Key terms

- [molar volume](https://www.gradenine.co.uk/glossary/molar-volume)
- [ideal gas equation](https://www.gradenine.co.uk/glossary/ideal-gas-equation)
- [real gas](https://www.gradenine.co.uk/glossary/real-gas)
- [high pressure](https://www.gradenine.co.uk/glossary/high-pressure)

## Related

- [Revision notes for IB DP Chemistry 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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