Equilibrium constants in real gas systems fugacity

The equilibrium constant defined by eqn. (26) can be used to calculate the equilibrium conversion of reactants to products under specified conditions of temperature and pressure. The activity of a component X in a mixture of ideal gases, Ox, is given by 

In defining activities in a real gas mixture, the partial pressure of each component is replaced by its fugacity, fx 

Fugacity is a thermodynamic property related to the deviation of the p—V—T properties of the gas from those of an ideal gas. At very low pressures, the fugacity of a real gas tends to its partial pressure 

In the absence of experimental measurements, it is usual to calculate the fugacities of components in a real gas mixture using the Lewis and Randall rule 

So far, only gaseous reactants have been considered. If solids are involved, the terms arising from the activities of the solids are usually omitted and the equilibrium constant includes only the fugacities of the gaseous reactants, e.g. for the reaction 

---