Question

On dissolving 0.5 g of a non-volatile non-ionic solute to 39 g of benzene, its vapour pressure decreases
from 650 mm Hg to 640 mm Hg. The depression of freezing point of benzene (in K) upon addition of the
solute is
(Given data: Molar mass and the molal freezing point depression constant of benzene are 78 g mol⁻¹ and
5.12 K kg mol⁻¹ , respectively)

Answer 1

Thus the depression of freezing point of benzene is ΔTf = 1.02

Explanation:

P0 − PS / PS = i [n Solute / n solvent]  

650 − 640 / 640 = 1 × 0.5 × 78 M × 39

⇒ M (Solute) = 64 gm

ΔTf = Kf × m = 5.12 × 0.5 × 1000 / 64 × 39

⇒ ΔTf = 1.02

Thus the depression of freezing point of benzene is ΔTf = 1.02