Two open beakers one containing a solvent and the other containing a mixture of that solvent with a non voltatile solute are together selated in a container. Over time :

Solution

Chemistry ma’am · 4 steps

The pure solvent has the higher vapour pressure, so its vapour keeps condensing into the solution.

1. Compare vapour pressures. By Raoult's law, a non-volatile solute lowers the vapour pressure of the solvent: p = xₛₒₗᵥₑₙₜ p°. Since xₛₒₗᵥₑₙₜ < 1, the solution has a lower vapour pressure than the pure solvent.

   pₛₒₗᵤₜᵢₒₙ = xₛₒₗᵥₑₙₜ p° < p°

2. Shared vapour space. In the sealed container both beakers share one vapour. The vapour pressure that keeps the pure solvent in balance is higher than what the solution can hold, so vapour keeps condensing into the solution.
3. Net transfer. The pure solvent keeps evaporating to make up the loss. So solvent moves, through the vapour, from the pure-solvent beaker into the solution beaker.
4. So. The volume of the solution increases and the volume of the pure solvent decreases. The answer is (a).

The idea to keep. Vapour always flows from the higher vapour pressure to the lower one. A solution with a non-volatile solute is the lower one, so it pulls solvent towards itself.

Answer from the NTA final answer key. Solution by PrepWiser.