Vapour Pressure of a Solid-Liquid Solution:
When a non-volatile solid is added to the solvent to form a solution, then the vapour pressure of the solution is found lower than the vapour pressure of the pure solvent at the same temperature. The decrease in the vapour pressure of the solution solely depends on the quantity of non-volatile solute present in solution. The vapour pressure of such solution is given by using the general equation of Roult’s Law.
Assume that water is component 1 and a non-volatile component is component 2, then vapour pressure of the solution will be equal to the vapour pressure of the solvent in the solution. Vapour pressure of solvent p1 is proportional to its mole fraction in solution and given as: