Question

The depression of freezing point of a solution of acetic acid in benzene is – 0.2°C. If the molality of acetic acid is 0.1 m, then find the ratio of the normal mass to the abnormal mass. (Assume Kf of acetic acid = 4.0°C m^-1)

(a) 1.5

(b) 0.8

(c) 0.5

(d) 0.2

The question was posed to me at a job interview.

Origin of the question is Abnormal Molar Masses topic in chapter Solutions of Chemistry – Class 12

Answer 1

The correct choice is (c) 0.5

Easy explanation: Given,

Depression in freezing point(ΔT) = -0.2 °C

Molality of the solution(m) = 0.1 m

Freezing point depression constant (Kf) = 4.0 °C m^-1

Let the Van’t Hoff factor = i

We know that, ΔT = i x Kf x m

  0.2 = i x 4 x 0.1

  i = 0.2/0.4 = 0.5

Therefore, the ratio between the normal mass and abnormal mass, which is equal to the Van’t Hoff factor is equal to 0.5.