Molal elevation constant:
Molal elevation constant can be defined as the elevation in boiling point produced when one mole of solute is dissolved in 1 kg i.e. 1000 g of the solvent. It is also known as the ebullioscopic constant.
If T0b is the boiling point of the pure solvent and Tb is the boiling point of the solution then elevation in boiling point is given as
∆Tb =T0b-Tb
Experimental results show that there is a relation between elevation in boiling point and molality ‘m’ of the solute present in solution
∆Tb ∝ m
∆Tb = kb m
Where,
kb = molal elevation constant
Substituting the value of ‘m’ in the above relation we get
∆Tb = 1000 x kb x m2 / M2 x m1
Where,
m2 = mass of solvent in g
M1 = mass of solvent in kg
M2 = molar mass of solute
Molal depression constant:
The freezing point depression of solutions of nonelectrolytes has been determined to be equal to the molality (m) of the solute times a proportionality constant called the molal freezing point depression constant
If T0f is the boiling point of the pure solvent and Tf is the boiling point of the solution then depression in the freezing point is given as
∆Tf =T0f-Tf
Just like elevation in boiling point, depression in freezing point is also directly related to molality ‘m’.
∆Tf = 1000 x kf x m2 / M2 x m1
Where,
k f = molal depression constant
m2 = mass of solvent in g
M1 = mass of solvent in kg
M2 = molar mass of solute
