Chemical and Physical Equilibrium in Two Phases

If the two vapor pressures were equal, then Eqs. 13.9 and 13.BP would predict the same mol fractions in the liquid as in the gas. In reality, the two vapor pressures are similar, but not the same (NBP s are —0.6 and — 10°C) however, Eq. 13.10 is still obeyed. We may see this by replacing Eq. 13.10 by the same equation in terms of the fugacities. 

Why bother with this calculation There are very few industrial examples of reactions that come to equilibrium simultaneously in two phases, but 

When we discuss the phase rule (Chapter 15) we will refer back to this example. 

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