Depression in freezing point, means when the solvent is treated with non volatile solute, the vapour pressure of this solution lowered and the freezing point also decreases. So, we can say that vapour pressure is directly proportional to freezing point.
From depression in freezing point,
We can say that ∆Tf = Kf m (here m is molality of solution)
And Kf is the cryoscopic constant or we can call it molal depression constant.
Unit of Kf is Kelvin Kg / mol
Define Kf ,
∆Tf = Kf when the molality of solution is 1. Means when 1 mole of solute is added in 1 gram solvent the molality of solution is 1.
Molality (m) = No. Of moles of non-volatile solute / Mass of solvent in kg
Freezing point means the temperature at which the vapour pressure of liquid state becomes equal to vapour pressure of solid state.