Vapor pressure is a key observable of the solvent mole fraction
There is a direct coerelation between the vapor pressure above a solution and the mole fraction of the solvent. This relationship is called Raoult's law. It is also the basis for the theory of ideal solutions. The theory of ideal solutions provides a first example of a two-component phase diagram. Such a diagram has a two phase region, in which the solution is in equilibrium with the vapor, but each has a different composition. The Raoult's law theory predicts this behavior. The two-phase region observed in the pressure-component phase diagram is a general feature of phase diagrams. One can read the mole fractions of the two phases using a tie line that connects the two phases at a given value of the total pressure.
The tie line is used to connect two phases. THe lever rule is a method for calculating the relative amount of each phase present for a given total composition. The method is described in the video below.
