1. The problem statement, all variables and given/known data I'm trying to calculate the saturation pressure and the densities of a liquid and vapor using an equation of state. I figured out how to do it and know how to proceed except that I don't understand Maxwell's Principle which is stated in number 2 below. 2. Relevant equations ∫P.dV - Psat (Vg - Vl) = 0 integration limits of the ∫P.dV are from Vl to Vg Where P is the pressure, Psat is the saturation pressure, Vg is the gas volume, Vl is the liquid volume 3. The attempt at a solution I don't understand how principle is derived. The only equation which comes to mind that relates Pressure and Saturation Pressure is Raoult's Law which is y*P=x*Psat (y is the gas mole fraction and x is the liquid mole fraction). I would also appreciate it if I can be guided to some references which explains this prinicple, the book lacks this principle.