111. Define vapour pressure of a liquid. What happens to the vapour pressure of a liquid when (a) a volatile solute dissolves in it and (b) a non-volatile solute dissolves in it?

Answer:

Vapor pressure or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system.

(a) When a volatile solute is dissolved in a liquid total vapour pressure is given by the sum of total vapour pressure of both the solvent and solute ie.,
P = P_A + P_B.

(b) When a non-volatile solute is dissolved in a liquid, the vapour pressure of liquid decreases and this lowering of vapour pressure is proportional to the mole fraction of solute added to it.

$\frac{{\mathrm{P}}_{\mathrm{A}}^0-{\mathrm{P}}_{\mathrm{A}}}{{\mathrm{P}}_{\mathrm{A}}^0} = {\mathrm{X}}_{\mathrm{B}}$