Depression Of Freezing Point By A Non Volatile Solute

Depression Of Freezing Point By A Non Volatile Solute

The temperature at which solid and liquid states of a substance have the same vapour pressure is called freezing point. If a non volatile solute is mixed with a pure solvent, the freezing point of pure solvent always greater than the impure one.

where Tf₀ is the freezing point of the pure solvent and Tf is the freezing point of a non – volatile solute containing solvent and ΔTf is the depression in freezing point.

ΔTf ∝ molality

or ΔTf = Kf

Kf = molal freezing point depression constant of the solvent (cryoscopic constant)

m = molality of the solution

Molal freezing point depression constant of the solvent or cryoscopic constant is defined as the depression in freezing point which may be theoretically produced by dissolving 1 mole of any Non volatile non electrolyte solute in 1000 gm of solvent.

or, 

Where m₁ = molecular weight of solute

w = weight of solute

W = weight of solvent