The most convenient way of expressing the composition of a generic mixture is by using the mole fractions  (written  in the gas phase) of the different components (or chemical species: atoms or molecules) present in the system, where
: with , the number of moles of the component i, and , the total number of moles of all the different components present in the mixture. 
The standard state of each component in the mixture is taken to be the pure substance, i.e. the pure substance has an activity of one. When activity coefficients are used, they are usually defined in terms of Raoult's law,
where  is the Raoults law activity coefficient: an activity coefficient of one indicates ideal behaviour according to Raoults law.