CBSE Class 12-science Answered

DEPRESSION OF FREEZING POINT :

• A solution will freeze when its vapour pressure equals the vapour pressure of the pure solid solvent.
• According to Raoult’s law, when a non-volatile solid is added to the solvent its vapour pressure decreases and now it would become equal to that of solid solvent at lower temperature. Thus, the freezing point of the solvent decreases.

• The depression in freezing point depends on the number of solute molecules rather than their nature hence, it is a colligative property.
• Let T_f⁰ be the freezing point of pure solvent and T_f be its freezing point when non-volatile solute is dissolved in it. The decrease in freezing point ΔT_f⁰ = T_f⁰ – T_f  is known as depression in freezing point.

For dilute solutions, the depression in freezing point (ΔT_f) is directly proportional to the molal concentration of the solute in a solution (Molality). Thus,

Where,

      m = molality i.e. the number of moles of solute dissolved in 1 kg of solvent.

      K_f = Freezing Point Depression Constant or Molal Depression Constant or Cryoscopic Constant

The unit of K_f is K kg mol-1