ice and water are in equilibrium. When a nonvolatile solute is added to water at its freezing point, the freezing point of the solution is lowered from ˚C. The lowering of the freezing point of the solvent when a solute is added is called depression in freezing point (ΔT f ). From the above graph, we infer that the freezing point (T f o ) is ˚C as the vapour pressure at this temperature is atm (atmospheric pressure).
The vapour pressure versus temperature curve for the solution indicates that the freezing point (T f ) is lower than the ˚C. The depression in freezing temperature (ΔT f ) can be expressed as, ΔT f = T f o - T f The experimental results show that the depression in freezing point is directly proportional to the molal concentration of the solute particles. Hence, ΔT f α m ΔT f =K f m ........( . ) Here, ‘m’ = is the molality of the solution K f = molal freezing point depression constant or cryoscopic constant.
If m= then ΔT f =K f The K f is equal to the depression in freezing point for molal solution Table . Molal freezing point depression constant for some solvents S. No. Solvent Freezing point (K) K f (K.kg.
Ethanol . . . Benzene .
. Carbon disulphide . . .
Ether . . . Cyclohexane .
Determination of molar mass of solute from depression in freezing point If the solution is prepared by dissolving w B g of solute in w A g of solvent. then depression in freezing point is given by ( . ) ∆ T w M w f f A ............... ( .
) molar mass of a solute can be calculated using ( . ) M w T w f f A ∆ ............... ( . ) Example Problem - Ethylene glycol (C H O ) can be used as an antifreeze in the radiator of a car.