A solute in dilute solution usually follows Henrys law rather than Raoults law, and it is more usual to express the composition of the solution in terms of the molar concentration  (in mol/L) or the molality  (in mol/kg) of the solute rather than in mole fractions. The standard state of a dilute solution is a hypothetical solution of concentration  = 1 mol/L (or molality  = 1 mol/kg) which shows ideal behaviour (also referred to as "infinite-dilution" behaviour). The standard state, and hence the activity, depends on which measure of composition is used. Molalities are often preferred as the volumes of non-ideal mixtures are not strictly additive and are also temperature-dependent: molalities do not depend on volume, whereas molar concentrations do.
The activity of the solute is given by: