how is the vapour pressure of a solvent affected when a non volatile solute is dissolved in it ?
Vapour pressure of a liquid at any temperature can be defined as the pressure exerted by the vapours of the liquid in equilibrium with the liquid itself. The vapour pressure of a pure solvent decreases /lowers when a non- volatile solute is added to the solvent. This is because, on adding the solute, some of the solute molecules also occupy the surface hence the number of the solvent molecules present on the surface would decrease as compared to pure solvent. Since the number of solvent molecule on the surface is decreases it will decrease the number of evaporating molecules on surface which results in lowering of vapour pressure.
As shown in diagram below.
In first case there was only solvent molecule. In second case some of the solute molecule also present as the surface, decreasing the total number of solvent molecules on surface.
As shown in diagram below.
In first case there was only solvent molecule. In second case some of the solute molecule also present as the surface, decreasing the total number of solvent molecules on surface.
