In pure solvent the surface is oC Cupied by solvent molecules only. Number of molecules evpoarating will be more, hence vapour pressure is more. In case of solution the surface is oC Cupied by both solute and solvent molecules. The number of molecules evaporating will be less in case of solution containing non volatile solute. Hence the vapour pressure of solution containing non volatile solute is less than the vapour pressure of pure solvent.
