Summary

Dalton's law provides a method by which the total pressure of a gas mixture can be calculated using the partial pressures of the component gases of the mixture.

Definitions

:Pressure
:Partial pressure
:Quantity (moles)


Partial Pressure

Central to Dalton's law is the concept of partial pressure. Partial pressure is the hypothetical pressure of a single component of a gas mixture if it occupied the same volume as the total mixture and was at the same temperature. I.e. the pressure of the component after all other mixture components are removed from a fixed container and the subject component expands to fill the container.

When mixture components can be considered as ideal gases the partial pressure can be equated to volume and quantity (moles) as follows:

The partial pressure of a gas is a measure of its thermodynamic activity. Therefore for 'real' gases characterisation of partial pressures is more difficult as molecule size, diffusion characteristics and intermolecular forces come into play.

Dalton's Law

Dalton's law states that the total pressure of a non-reactive gas mixture is equal to the summation of the partial pressures of all its component gases:

Dalton's law relies on the ideal gas assumption, which is generally accurate for gases at low to medium temperatures and pressures, meaning Dalton's law is applicable for many practical situations.

Example

For an approximate simple mixture of air in a given container at atmospheric pressure with the composition:

Gas Mole Fraction Partial Pressure (kPa)
Nitrogen 78% 79.014
Oxygen 21% 21.273
Argon 1% 1.013
Total 100% 101.3

Here the total pressure is the sum of the partial pressures:



Further Reading

  1. Chemical Engineering Volume 2, Fifth Edition
  2. Perry’s Chemical Engineers’ Handbook, Eighth Edition
  3. Marks' Standard Handbook for Mechanical Engineers 11th Edition