Define Solvent Chemical Potentials from Phase Equilibria?

Previously explained how we can evaluate the activity coefficient γ_m,B of a nonelectrolyte solute of a binary solution if we know the variation of the osmotic coefficient of the solution from infinite dilution to the molality of interest. A similar procedure for the mean ionic activity coefficient of an electrolyte solute was described earlier.

The physical measurements needed to find the osmotic coefficient Φ_m of a binary solution must be directed to the calculation of the quantity μ^*_A -μ_A, the difference between the chemical potentials of the pure solvent and the solvent in the solution at the temperature and pressure of interest. This difference is positive, because the presence of the solute reduces the solvent's chemical potential.

To calculate Φ_m from μ^*_A -μ_A we use

Φ_m = μ^*_A -μ_A / RTM_Avm_B

where v for a nonelectrolyte is 1 and for an electrolyte is the number of ions per formula unit.