Depression in freezing point ### Definition of Molecular or molar depression constant-

If w/m = 1 mole, W =100 gram then ΔTf = Kf

### Definition of  molal depression constant-

If w/m = 1 mole, W =1000 gram then ΔTf = Kf

### ” 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,

ΔTf = molality x Kf

Unit of Kf = K molal -1   OR     K Kg mole -1

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

Kf =  M R Tf2/1000 ΔH fusion

Tf = 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= ΔH fusion /M

Kf  =R T2 /1000 Lf

Lf = 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).