This is the equation-of-state function derived from G(T,P,N1,N) by differentiation with respect to N1. Temperature is again fixed to give the equimolar critical-point composition. As expected, the surface forms a cusp catastrophe and covers the pressure range 1.1 > Pr > 0.8. The red-yellow-boundaries give states at the limit of stability as defined by the first diffusional stability criterion:
Note that when the data set is specified as N1, N2, , Nn-1, N rather than N1, N2, , Nn, the correct chemical-potential variable is
Scaling is then carried out using the total mole number N so as to give molar energies, molar entropies, etc., and compositions in mole fractions.