Question

On addition of non-volatile potassium iodide in water at 298K it is noticed that vapor pressure reduces from 23.8 mm Hg to 2.0 cm Hg. What is the mole fraction of solute in the solution?

(a) 0.916

(b) 0.160

(c) 0.084

(d) 0.092

I got this question during an online exam.

I would like to ask this question from Colligative Properties and Determination of Molar Mass in section Solutions of Chemistry – Class 12

Answer 1

The correct answer is (b) 0.160

The best I can explain: Given,

P^0water =23.8 mm Hg

Pwater= 2.0 cm Hg = 20.0 mm Hg (after addition of solute)

From the law of relative lowering of vapor pressure, ∆p1 = X2 x p^01 (where X2 is the mole fraction of solute)

On rearranging, Δp1/p^01 =X2

Δp1 = 23.8 – 20.0 = 3.8 mm Hg

X2 = 3.8/23.8 = 0.160.