Thermodynamic Excess Functions. Solutions of a Single Electrolyte

Thermodynamic Excess Functions. Solutions of a Single Electrolyte 

As composition variables we use molal solute concentrations m. The piolality nts of solute s is the ratio of the number of moles of s to the number of kilograms of solvent. For a single electrolyte with ions of species a and b the independent thermodynamic variables are T, P, nta, and mi,. In any partial derivative with respect to one of them, e.g., dG/dnia, all of the others are held fixed. 

We also introduce the notation m (without a subscript) for the sum of all solute molalities. In partial differentiations with respect to m the solute mole ratios such as mjmi, are held fixed. 

The Gibbs free energy per kilogram of solvent for a single electrolyte may be written as 

The osmotic coefficient which is analogous to the compressibility factor 

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