Depression in Freezing Point - Practice Questions & MCQ

Depression in Freezing Point:

Freezing Point: It is the temperature at which the liquid and the solid form of the same substance are in equilibrium and have the same vapour pressure. A solution freezes when its vapour pressure is equal to the vapor pressure of pure solid solvent. Due to lower vapour pressure of the solution, solid form of a solution separates out a lower temperature.

On adding a non-volatile solute to the solvent, the vapor pressure of the solution is lesser than the solvent and the vapor pressure of the solution becomes equal to the vapor pressure of solid solvent at a lower temperature when compared to the pure solvent and hence the freezing point of decreases.

Suppose are the freezing point of pure solvent and solution respectively. Decrease in freezing point  is given as:

• This is also termed as cryoscopy and depression of freezing point
• For a dilute solution,  is directly proportional to the molality (m) of the solution.

            Hence 
            
            If molality of the solution is one, then
              
             and M can be found out by using these relations.

Here w = weight of solute
W = weight of solvent
K_f = molal depression constant or cryoscopic constant.
M = molar mass of solute

M1 = molar mass of solvent

The value of K_f or Cryoscopic constant is a property of the solvent only and does not depend on the type of solute. The value of K_f can be calculated as: 

Here,