249. A reasonably general expression for vapour-liquid phase equilibrium at low to moderate pressure is Φ

_{i}y_{i}P = Y_{i}x_{i}f_{i}° where, Φ is a vapor fugacity component, Y_{i}is the liquid activity co-efficient and f_{i}° is the fugacity of the pure component i. the K_{i}value (Y_{i}= K_{i}x_{i}) is therefore, in general a function ofPage **36** of **75**