Question

How can you justify the observation that the vapour pressure of a solution of a non-volatile solute in a given solvent is less than that of the pure solvent? Also state the law related to this observation.

Answer 1

The observation that the vapour pressure of a solution containing a non-volatile solute is less than that of the pure solvent can be justified by the fact that the non-volatile solute does not evaporate, and it reduces the number of solvent molecules available at the surface to escape into the vapour phase. As a result, fewer solvent molecules evaporate, leading to a lower vapour pressure than that of the pure solvent.

This observation is explained by Raoult’s Law, which states that:

`P_"solution" = chi_"solvent" * P_"solvent"^circ`

Where,

`P_"solution"`​ is the vapour pressure of the solution,

`chi_"solvent"` is the mole fraction of the solvent,

`P_"solvent"^circ`​ is the vapour pressure of the pure solvent.

Since `chi_"solvent"` < 1 in a solution, the vapour pressure `P_"solution"`​ < `P_"solvent"^circ`​