Because of the missing intermolecular forces, no volume of mixing occurs and we have to consider only the ideal entropy of mixing 
. It is convenient to incorporate the entropy of mixing in to the last term of Equation (23)
(24)
Differentiation using Equation (6) gets the respective µi for each component i:
(25)
So the fugacity coefficient ϕim in the mixture is defined as:
(26)
The evaluation of 
requires an EOS but now for fluid mixtures. Using Equation (26) and we get the general expression:
(27)
Or:
(28)
