Raoult’s Law

n The quantitative relationship between vapor-pressure lowering and concentration in an ideal solution is stated in Raoult’s Law: the partial vapor pressure of a component in solution is equal to the mole fraction of that component times its vapor pressure when pure at a temperature;

$${\rm P}_1 = {\rm X}_1 {{\rm P}^{\rm o}}_1 $$

Where P₁ and P^o ₁ are the vapor pressure of the solution and the pure solvent, respectively, X₁ is the mole fraction of solvent, X₁ = 1−X₂ and P₁ = (X₁−X₂)P^o ₁, and,

$${\rm X}_2 = {{{{{\rm P}^{\rm o}}_1} - {\rm P_1 }} \over {{{\rm P}^{\rm o}}_1}}$$

this means that the fractional vapor–pressure lowering is equal to the mole fraction of the solute; also, the total pressure of the system is equal to the sum of the partial pressures,

$${\rm P}_{\rm s} = {\rm P}_1 + {\rm P}_2 +{\rm P}_2 \ldots {\rm P}_{\rm n} .$$

(Atkins P, de Palua J (2009) Physical Chemistry. W. H. Freeman & Company, New York).