Solutions

A solution of two miscible liquids showing negative deviation from Raoult's law will have :

1. increased vapour pressure, increased boiling point
2. increased vapour pressure, decreased boiling point
3. decreased vapour pressure, decreased boiling point
4. decreased vapour pressure, increased boiling point

Answer

(4) decreased vapour pressure, increased boiling point

Solution

Negative deviation: A–B attractions are stronger than A–A and B–B

⇒ fewer molecules escape ⇒ ptotal < pA0xA + pB0xB ⇒ vapour pressure decreases

∵ a lower vapour pressure reaches the external pressure only at a higher temperature ⇒ boiling point increases

(e.g. chloroform + acetone, maximum boiling azeotrope)

Answer: (4)