Depression in freezing point
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Definition of Molecular or molar depression constant-
If w/m = 1 mole, W =100 gram then ΔTf = Kf ‘
” 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 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)
Lf = ΔH fusion /M
Kf =R Tf 2 /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).
