When a non-volatile substance is dissolved in a liquid, the vapour pressure of the liquid (solvent) is lowered. According to Raoult’s law (1887), at any given temperature the partial vapour pressure (pA) of any component of a solution is equal to its mole fraction (XA) multiplied by the vapour pressure of this component in the pure state .
The vapour pressure of the solution is the sum of the partial pressures of the components, i.e., for the solution of two volatile liquids with vapour pressures and .
Alternatively, Raoult’s law may be stated as “the relative lowering of vapour pressure of a solution containing a non-volatile solute is equal to the mole fraction of the solute in the solution.”
Relative lowering of vapour pressure is defined as the ratio of lowering of vapour pressure to the vapour pressure of the pure solvent. It is determined by Ostwald-Walker method.
Thus according to Raoult’s law,
where, Vapour pressure of the solution
Vapour pressure of the pure solvent
Number of moles of the solute
Number of moles of the solvent
and weight and mol. wt. of solute
and weight and mol. wt. of the solvent.
Limitations of Raoult’s law
Raoult’s law is applicable only to very dilute solutions.
Raoult’s law is applicable to solutions containing non-volatile solute only.
Raoult’s law is not applicable to solutes which dissociate or associate in the particular solution.