Generally, concentrations of substances should not be expressed in mole fractions. Formula on excess Gibbs free energy of 1 mole of solution of solute B in solvent A, for
EHL
model, assuming pure substance state as standard state, is then:
From foregoing formula on logarithm of solvent A activity coefficient may be obtained:
and formula on logarithm of solute B activity coefficient may be obtained:
that is two-parameter equation on solvent A activity coefficient and three-parameter equation on solute B activity coefficient, leading to four parameters in the whole range of concentrations. The quantity of parameters may be reduced to two, for example advantaging of activity coefficient B in infinite dilution value.