Question

On adding a non volatile solute to a volatile solvent:

• A. Vapour pressure of the solution increases.
• B. Vapour pressure of the solution decreases.
• C. Freezing point of the solution increases.
• D. Freezing point of the solution remains unchanged

Answer 1

The correct option is B Vapour pressure of the solution decreases.

On adding non volatile solute to a volatile solute , vapour pressure of the solution reduces.
This lowering in vapour pressure is due to the fact that after the solute was added to the pure liquid (solvent), the liquid surface now has molecules of both, the pure liquid and the solute. The number of solvent molecules escaping into vapour phase gets reduced and as a result the pressure exerted by the vapour phase is also reduced. This is known as relative lowering of vapour pressure.

On adding non volatile solute to a volatile solute, freezing point of the solution decreases.
Freezing point: The temperature at which the vapour pressure of the substance in its liquid phase is equal to its vapour pressure in the solid phase.
On addition of non-volatile solute in pure liquid, the F.P. of the solution becomes lower than that of pure liquid.
At freezing point, the VP of a substance in liquid phase is equal to its VP in solid phase
$V{P}_1=V{P}_2$
Since, the vapour pressure reduces on adding a non volatile solute,
Now the V.P. would become equal to that of solid solvent at lower temperature
So adding non volatile solute will decrease the F.P. of solvent by $\Delta T_f$
$\Delta T_f=T_f-T_f^{\circ }$
$T_f^{\circ }=\text{Freezing point of pure liquid}$
$T_f=\text{Freezing point of solution}$