Question

How do you calculate freezing point depression?

Answer 1

Freezing point: Freezing point is defined as the temperature at which vapour pressure of a substance in the liquid phase is equal to vapour pressure in the solid phase.

Depression in freezing point:

• When a non-volatile solute is added to the volatile solvent, the vapour pressure of the volatile solvent decreases.
• Due to a decrease in vapour pressure of the liquid phase, it becomes equal to the vapour pressure of solid at a lower temperature.
• The difference in the freezing point of pure solvent and that of the solution is known as depression in the freezing point.

$∆{\mathrm{T}}_{\mathrm{f}}=\mathrm{Freezing}\mathrm{point}\mathrm{pure}\mathrm{solvent}-\mathrm{Freezing}\mathrm{point}\mathrm{of}\mathrm{solvent}\mathrm{after}\mathrm{addition}\mathrm{of}\mathrm{solvent}$

Calculation of freezing point depression:

• For a dilute solution, the depression in the freezing point is directly proportional to the molality of the solute.

$∆{\mathrm{T}}_{\mathrm{f}}\propto \mathrm{m}$

$∆{\mathrm{T}}_{\mathrm{f}}=\mathrm{i}\times {\mathrm{K}}_{\mathrm{f}}\times \mathrm{m}$

• where, $∆{\mathrm{T}}_{\mathrm{f}}$ = Freezing point depression, $\mathrm{m}$= molality of solute added, ${\mathrm{K}}_{\mathrm{f}}$ = Cryoscopic constant
• Unit of Freezing point depression is $\mathrm{K}$or $\mathrm{Kelvin}$