Fugacities

Let us start from the definitions of Helmholtz and Gibbs free energies *A* and *G* using the inner energy *U*, the enthalpy *H* and the entropy *S*.

(1)

(2)

In the following the partial derivatives of *A* and *G* in respect to *P* and *T* at constant composition *n**i*

(3)

(4)

(5)

and the chemical potentials *µ**i*

(6)

are used.