7.1.5—The terms mole fraction and partial pressure

Mole fraction is the moles of one gas divided by total moles. For an ideal gas mixture, partial pressure pᵢ = xᵢP_total, where xᵢ is the mole fraction.

The partial pressures add to the total pressure. Use mole fraction rather than mass fraction because gas behaviour in the ideal model is tied to particle amount.

A mixture with 2 mol N₂ and 1 mol O₂ has x(N₂)=2/3. At total pressure 300 kPa, p(N₂)=200 kPa.

Partial pressure is not the pressure of a separated sample at the same volume. It is the component’s contribution within the mixture.