Depression in Freezing Point MCQ - Practice Questions with Answers

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 it's vapour pressure is equal to the V.P 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 it's vapour pressure decreases and becomes equal to the V.P. of solid solvent at lower temperature hence the freezing point of solvent also decreases.
Suppose T^o_f T_f are the freezing point of pure solvent and solution respectively than decrease in freezing point ΔT is given as:

• This is also termed as cryoscopy and depression of freezing point (ΔT_f)
• It can be measured by Beckmann's thermometer method and Rast's method.
• For a dilute solution, ΔT_f is directly propotional to the molality (m) of the solution.
  Hence ΔT_f ∝ m
            ΔT_f = K_f m
  If molality of the solution is one, then
  ΔT_f = K_f
  ΔT_f and M can be found out by using these relations.
  
  
  
  
  
  

NOTE: The value of K_v or K_f depends on only nature of solvent and not on the nature of the solute.