In a mixture of gases, the fugacity of each component  has a similar definition, with partial molar quantities instead of molar quantities (e.g.,  instead of  and  instead of ):
and
where  is the partial pressure of component . The partial pressures obey Dalton's law:
where  is the total pressure and  is the mole fraction of the component (so the partial pressures add up to the total pressure). The fugacities commonly obey a similar law called the Lewis and Randall rule:
where  is the fugacity that component  would have if the entire gas had that composition at the same temperature and pressure. Both laws are expressions of an assumption that the gases behave independently.