(1) Relative vapour pressure of a solution (2) Lowering of vapour pressure of a solution (3) Relative lowering of vapour pressure.

Question

Explain the following terms : 

(1) Relative vapour pressure of a solution 

(2) Lowering of vapour pressure of a solution 

(3) Relative lowering of vapour pressure.

Answer 1

(1) Relative vapour pressure of a solution : If P₀ is the vapour pressure of a pure liquid (solvent) and P is the vapour pressure of a solution after adding a nonvolatile solute, then, relative vapour pressure = \(\frac{P}{P_0}\).

(2) Lowering of vapour pressure of a solution : When a nonvolatile solute is added to a pure solvent, the surface area is covered by the solute molecule decreasing the rate of evaporation, hence its vapour pressure decreases. This decrease in vapour pressure is called lowering of vapour pressure. If P₀ is the vapour pressure of a pure solvent (liquid) and P is the vapour pressure of the solution, where P < P₀, then, (P₀ – P) is the lowering of the vapour pressure.

(3) Relative lowering of vapour pressure : If P₀ and P are the respective vapour pressures of a pure liquid (solvent) and the solution containing a non-volatile solute then P < P₀. 

Hence, 

P₀ – P represents the lowering of the vapour pressure due to addition of a nonvolatile solute.

∴ Relative lowering of vapour pressure = \(\frac{P_0-P}{P_0}\) = \(\frac{ΔP}{P_0}\)