The Effect of an Inert Gas on Vapor Pressure

Liquid-vapor equihbria commonly are observed when the system is exposed to the atmosphere (as in Fig. 8.2) rather than only to the vapor itself Therefore, it is useful to derive the equations that are applicable to such a simation. We will assume that air is essentially insoluble in the liquid phase and that atmospheric pressure is represented by P. The saturation vapor pressure of the liquid in the absence of any foreign gas such as air can be shown to differ from that of the partial pressure of the vapor p in the presence of air. 

As the liquid and vapor are in equilibrium at a given temperature and total pressure P, we can write 

Let us assume that the vapor behaves as an ideal gas, even in the presence of the foreign gas. From Equation (7.43), we see that for one mole of an ideal gas at constant T 

Integration of this equation from the vapor pressure of the liquid p to the standard state for an ideal gas, a pressure of 0.1 MPa, P°, 

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