Relative lowering of vapour pressure is directly proportional to the mole fraction of solute. Hence, it is a colligative property.
The law was discovered by the French chemist Francois Raoult (1830–1901). It states that 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 (the number of moles of solvent divided by the 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.