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