The partial vapour pressure of a solvent is proportional to its mole fraction. If p is the vapour pressure of the solvent (with a substance dissolved in it) and X the mole fraction of solvent (number of moles of solvent divided by total number of moles) then p = p0X, where p0 is the vapour pressure of the pure solvent. A solution that obeys Raoult's law is said to be an ideal solution. In general the law holds only for dilute solutions, although some mixtures of liquids obey it over a whole range of concentrations. Such solutions are perfect solutions and occur when the intermolecular forces between molecules of the pure substances are similar to the forces between molecules of one and molecules of the other. Deviations in Raoult's law for mixtures of liquids cause the formation of azeotropes. The law was discovered by the French chemist François Raoult (1830–1901).
http://web.lemoyne.edu/~giunta/raoult2.html Raoult's original paper
Subjects: Chemistry — Physics.