Chemistry Twelve

Solutions

Depression of Freezing Point

Freezing point

Point at which the solid phase of a substance is in the dynamic equilibrium with the liquid phase is called freezing point.

In other words the temperature at which its liquid phase is equal to the vapour pressure in the solid phase, is called the freezing point.

The lowering of vapour pressure of a solution resulting in the lowering of freezing point compare to the the pure 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. This decreases in the freezing point of the solvent.

LetSolutions class 12 chemistryTf0 is the freezing point of pure solvent.

And Solutions class 12 chemistryTf is its freezing point when non-volatile solute is dissolved in it.

Therefore, the decrease in freezing point

Solutions class 12 chemistry28 .

This is called the depression in freezing point.

The depression of freezing point for dilute solution (ideal solution) is directly proportional to the molality ‘m’ of the solution.

Solutions class 12 chemistry29

Here Kf is the proportionality constant. This depends on the nature of the solvent. This is known as the Freezing Point Depression Constant or Molal Depression Constant or Cryoscopic Constant.

Unit of Solutions class 12 chemistry30

If w2 gram of the solute having molar mass equal to M2, present in w1 gram of solvent. And this produces the depression in freezing point Solutions class 12 chemistryDeltaTf of the solvent, then molality (m) of the solute can be expressed as

Solutions class 12 chemistry31

By putting the value of ‘m’ from equation (ii) in equation (i) we get

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Thus by knowing other quantities molar mass of the solute can be calculated using equation (iv)

The value of Solutions class 12 chemistry33depends upon the nature of the solvent can be determined from the following equations.

Solutions class 12 chemistry34

Where, R is gas constant

M1 is the molar mass of solvent.

Tf is freezing point of pure solvent in Kelvin.

Tb is the boiling point of pure solvent in Kelvin.

Solutions class 12 chemistryUntitled1are the ethanlpies of fusion and vapourization of the solvent respectively.

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