The Ideal Solution Approximation and Raoult’s Law

By our equilibrium constraint,

By our ideal solution approximation in both phases, the equilibrium criteria becomes

Now we need to substitute the expressions for f_i^V and f_i^L that we developed in Chapter 9. The fugacity of the pure vapor comes from Eqn. 9.25:

The fugacity of the liquid comes from Eqn. 9.39:

Combining Eqns. 10.70–10.72,

Writing in terms of the K_i ratio,

Note: at reasonably low pressures,

resulting in Raoult’s Law,