Question

If ethanol and chloroform are present in a molar ratio of 2:3 then what is the vapor pressure at 20° C if vapor pressures of pure liquids are 5.95 kPa and 21.17 kPa, respectively?

(a) 16.692 kPa

(b) 15.082 kPa

(c) 8.731 kPa

(d) 12.038 kPa

I got this question in an interview for internship.

My question is taken from Vapour Pressure of Liquid Solutions topic in chapter Solutions of Chemistry – Class 12

Answer 1

The correct answer is (b) 15.082 kPa

Easy explanation: Given,

P^0eth = 5.95 kPa

P^0chl = 21.17 kPa

Mole ratio of ethanol ∶ chloroform = 2 ∶ 3

Total number of parts = 2 + 3 = 5

Therefore, mole fraction of ethanol, Xeth = 2/5 = 0.4

Mole fraction of chloroform, Xchl = 3/5 = 0.6

From Raoult’s law, pA = p^0A x XA

Peth = 5.95 x 0.4 = 2.38 kPa

Pchl = 21.17 x 0.6 = 12.702 kPa

From Dalton’s law, Ptotal = Peth + Pchl

Ptotal = 2.38 + 12.702 = 15.082 kPa.