# Dalton's law of partial pressures

In a mixture of gases, each gas adds its own pressure as if it were alone. The total pressure is the sum of the partial pressures. Each partial pressure equals the mole fraction times the total pressure.

Chemistry (university) · Part 03: Intermolecular forces, gases and solutions · Card 5 of 9 · Last updated 2026-10-19

## Analogy

Several people talking in a room: the total noise is the sum of each voice, and each voice is unaffected by the others.

## Example

**Air at 1.00 atm**

Oxygen is 21% of the moles.
P(O₂) = 0.21 × 1.00 atm
P(O₂) = 0.21 atm

## In practice

Divers breathe special gas mixes. They watch the partial pressure of oxygen so it is neither too low nor harmful.

## Practice

### Flashcards

- **How do you find a gas's partial pressure?** Mole fraction × total pressure.

## Questions people ask

### What is mole fraction?

Mole fraction is the moles of one gas divided by the total moles of all gases. It has no units and the fractions in a mixture add up to 1. If a mixture has 3 mol of N₂ and 1 mol of O₂, then N₂ has a mole fraction of 0.75.

### Why do we correct for water vapour when collecting a gas over water?

A gas bubbled through water picks up water vapour, so the total pressure includes the vapour's pressure. To find the pressure of the dry gas, subtract the vapour pressure of water at that temperature from the total. Only then can you use PV = nRT to find the moles of the gas.

## Related concepts

- [Ideal gas law](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-3/chemistry-university-part-3-concept-4)
- [Kinetic molecular theory](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-3/chemistry-university-part-3-concept-6)
- [Real gases and van der Waals equation](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-3/chemistry-university-part-3-concept-7)

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