A non-volatile solute dissolved in a liquid, with a vapour pressure of the pure solvent higher than the vapour pressure of the solution.
In 1848 Von Babo and Wullner gave simple relation between the solution of the vapour pressure and concentration, but they failed. Then after 1982, Raoult performed several experiments and successfully gave Raoult’s law. The state that,
“The relative lowering of vapor pressure is equal to the mole fraction of the solute.”
Here, the vapour
pressure of pure solvent = P and Vapour pressure of pure solution = Ps, the relative lowering of vapour pressure is,
n1 = number of moles of solute and n2 = number of moles of solvent
For Raoult’s law
n1 is very small in comparison to n2 for very dilute solutions; therefore, n1 is neglected, and equation (1) we can write as,
Above equation (2) is the mathematical form of Raoult’s law.
Raoult’s law is used to calculate lowering vapour pressure, the number of moles of solute and solvent.
