Statistical Mechanics and the Vapor Pressure Curve

Statistical Mechanics and the Vapor Pressure Curve.—From statistical mechanics, we know how to write down the Gibbs free energy of the solid and perfect gas explicitly. All we need do, then, to find the complete equation of the vapor pressure curve is to equate these quantities. Thus, remembering that (dG/dT)P = — S, we can write the Gibbs 

Equation (6.4) is the general one for vapor pressure, and it shows that the undetermined constant in In P, in Eq. (5.9), is just the chemical constant that we have already determined in Eq. (3.16) of Chap. VIII. The simplest experimental method of finding the chemical constants is based on Eq. (6.4) one measures the vapor pressure as a function of the temperature, finds the specific heats of solid and gas, so that one can calculate the term in the specific heats, and computes the quantity 

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