Question

5 moles of liquid X and 10 moles of liquid Y make a solution having a total vapour pressure 70 torr. The vapour pressures of pure X and pure Y are 64 torr and 76 torr respectively. Which of the following is true regarding the described solution?

(a) The solution shows positive deviation

(b) The solution shows negative deviation

(c) The solution is ideal

(d) The solution has volume greater than the sum of individual volumes

I have been asked this question during a job interview.

The query is from Ideal and Non-Ideal Solutions topic in portion Solutions of Chemistry – Class 12

Answer 1

Correct choice is (b) The solution shows negative deviation

Explanation: Given,

Observed pressure = 76 torr

According to Raoult’s law,

pA = xA x pA^0 = 5/15 x 64 = 21.33 torr

pB = xB x pB^0 = 10/15 x 76 = 50.67 torr

Therefore, pressure expected by Raoult’s law = 21.33 + 50.67 = 72 torr.

Thus, observed pressure (70 torr) is less than the expected value. Hence, the solution shows negative deviation.