Question

If two liquids A and B form minimum boiling azeotrope at some specific composition then:
(A) A-B interactions are stronger than those between A-A or B-B.
(B) vapour pressure of solution increases because more molecules of liquids A and B can escape from the solution.
(C) vapour pressure of solution decreases because less number of molecules of liquids A and B can escape from the solution.
(D) show a large positive deviation from Raoult's law.

Answer 1

Hint: Raoult's Law gives a relationship between mole fraction and partial pressure of volatile liquids. Raoult’s law states that, ‘the mole fraction of solute and the partial pressure shows direct proportionality’.

Complete answer:
> When a minimum boiling azeotrope is formed, the resulting solution will have maximum vapour pressure as it has minimum boiling point. It suggests that the interactions between A-A or B-B are stronger than A-B interactions. Hence, the option (A) is ruled out.
> The total vapour pressure of solution is more than the sum of the partial pressures of two liquids A and B. This is due to weaker A-B interactions so that more molecules can escape the solution. Hence, the options (B) and (C) are ruled out.
> A positive deviation occurs from Raoult's law when the total vapour pressure of solution is more than the sum of the partial pressures of two liquids A and B.

Hence, the option (D) is the correct option.

Additional Information For a maximum boiling azeotrope, A-B interactions are stronger than those between A-A or B-B. Also, the total vapour pressure of solution is less than the sum of the partial pressures of two liquids A and B. This is due to stronger A-B interactions so that fewer molecules can escape the solution.
A Negative deviation occurs from Raoult's law when the total vapour pressure of solution is less than the sum of the partial pressures of two liquids A and B.

Note: An azeotrope is a constant boiling mixture. When an azeotropic mixture is boiled, its composition remains constant throughout. When the vapours are condensed, they have the same composition as the liquid mixture that is boiled.