Depression of freezing point
Depression of freezing point
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Freezing point of a substance is the temperature at which the vapour pressure of the substance in its liquid phase is equal to the vapour pressure in the solid phase.
If the vapour pressure of the solution is equal to the vapour pressure of the pure solid solvent then the solution freezes.
According to Raoult’ s law, addition of non-volatile solute decreases the vapour pressure of the solvent and would be equal to that of solid solvent at lower This decreases the freezing point of the solvent.
The decrease in the freezing point = ΔTf = Tf – Tf 0. This is known as depression in freezing point.
Tf 0 = freezing point of pure solvent
Tf 0 = freezing point when non-volatile solute is dissolved
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In a dilute solution the depression of freezing point Tf is directly proportional to molality of the solution. Mathematically,
ΔT f ∝m ΔT f = Kf m
Kf = Proportionality constant depending upon the nature of the solvent. This is known as Freezing Point Depression Constant or Molal Depression Constant or Cryoscopic Constant. The unit of Kf is K kg mol-1.
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Let w2 and M2 = Masses and molar masses of solute w1 = Mass of solvent
Putting this value of molality in the equation ΔT f = Kf m