The Fugacity Function of a Pure Real Gas

We expressed the molar free energy of a pure ideal gas as [Equation (10.10)] 

Similarly, we obtained the chemical potential of a component of an ideal gas mixture as [Equation (10.17)] from an analysis of the van t Hoff mixing experiment, using the same integral. 

An advantage of the fugacity over the chemical potential as a measure of escaping tendency is that an absolute value of the fugacity can be calculated, whereas an absolute value of the chemical potential cannot be calculated. 

As all gases approach ideahty as their pressure is decreased, and as Equation (10.29) is of the same form as Equation (10.14) for an ideal gas, it is convenient to complete the definition of/by stating 

From Equation (10.29), we can see that the change in the Gibbs function for the isothermal expansion of a real gas is 

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