Perfect Gases and Gaseous Mixtures

The activity of a perfect gas, as for any substance, is unitary, by definition, at standard state. Moreover, for a perfect gas, activity is (numerically) equivalent to pressure, at all pressures. Let us consider the relationship existing, with T held constant, between the chemical potential of component i in gaseous phase g at 1 bar (/a, 17 ) and at pressure P 

Because for a pure phase partial molar volume is equivalent to molar volume Vg, and since we have defined as standard state the condition of the pure component at P = 1 bar and T of interest  

In the more general case of a standard state pressure of reference different from 1, we have 

Interaction forces among gaseous molecules are generally subdivided into 

The first term on the right in the sum of potentials in equation 9.12 is the electrostatic term (represented here as a simple dipole moment). Factor / is the angular function of dipole orientation  

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