Vapor Equilibrium at Constant Pressure

In this section we consider a two-component, two-phase Uquid-vapor system. We assume that phase 1, the liquid, and phase 2, the vapor, are ideal. Hence, 022 = 1. 022 = and Lj, L2, Avi, and Av2 are independent of composition. Thus 

In a similar fashion, we arrive at the result Equation (9-78) can be written in terms of X2 and 2 in the form Substituting Eq. (9-79) into Eq. (9-77), and solving for X2, we find 

Equations (9-80) and (9-81) provide explicit expressions for and as functions of T and implicit expressions for T as functions of X2 and y2. Thus, temperature-composition diagrams similar to Fig. 9-6 can be constructed from knowledge of the boiling points and heats of vaporization at the boiling points of pure liquids 1 and 2. 

Equation (9-77) can be obtained in a more elementary fashion. The chemical potential of component 2 in an ideal-gas mixture is given by the expression 

The chemical potential of component 2 in an ideal-liquid mixture is given by the expression 

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