Depression in freezing point-Part 2

Depression in freezing point

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Definition of Molecular or molar depression constant-

If w/m = 1 mole, W =100 gram then ΔT_f = K_f ‘

” Molar depression constant is equal to the depression in freezing point which is produced when  one mole of nonvolatile ,non electrolyte solute is dissolved in 100 gram of solvent.

Definition of  molal depression constant-

If w/m = 1 mole, W =1000 gram then ΔT_f = K_f

” Molal depression constant is equal to the depression in f. p which is produced when  one mole of nonvolatile ,non electrolyte solute is dissolved in 1000 gram of solvent.

We know,

w x 1000/m W = molality

So,

ΔT_f = molality x K_f

Unit of K_f = K molal ^-1   OR     K Kg mole ^-1

Molal depression constant K_f for a particular solvent  can be calculated as ,

K_f =  M R T_f²/1000 ΔH _fusion

T_f = freezing point of solvent in Kelvin

M = Molecular weight of solvent

R = gas constant (in calorie),

ΔH _fusion = Latent heat of  fusion for one mole of frozen solvent (calorie /gram)

L_f = ΔH _fusion /M

K_f  =R T_f ² /1000 L_f

L_f = Latent heat of fusion per gram of solvent

IMPORTANT POINTS TO REMEMBER-

1) Depression of freezing point is a colligative property because it depends upon the molality of solution (means number of moles present in definite amount of solvent).

2) Equi molal solutions of different nonvolatile and non electrolyte substances dissolved in the same solvent show the same depression in freezing point (colligative property).