On the relation between activity coefficients and excess gibbs energy functions

The relation between activity coefficients and excess Gibbs energy functions for mixtures of electrolytes and nonelectrolytes is examined under a general framework. It is shown that the use of molality, instead of mole fraction, as a composition variable leads to unusual expressions for the activity coefficients of both solvents and solutes. Explicit expressions for activity coefficients in binary and multicomponent mixtures are derived and the importance of selecting proper standard states is highlighted.