The pressure exerted by a gas or vapour in a mixture at constant temperature that the gas would exert if it alone occupied the whole volume actually occupied by the mixture. The ratio of partial pressures in a mixture of gases is therefore the same as the ratio of volumes. The partial pressure of a component within a mixture of liquids is the product of the vapour pressure of the pure component and its mole fraction in the liquid phase. The partial pressure, p, of a component, i, of a gas mixture is directly proportional to the total pressure, p, where for an ideal gas:

where y_i is the mole fraction in the vapour phase.