Question

A liquid mixture having composition corresponding to point z in this figure shown is subjected to distillation at constant pressure. Which of the following statements is correct about the process?

(A) The composition of distillate differs from the mixture
(B) The boiling point goes on changing
(C) The mixture has highest vapour pressure than for any other composition
(D) Composition of an azeotrope alters on changing the external pressure.

Answer 1

Hint : It is mentioned that we have constant pressure. In an Azeotropic mixture, it has a characteristic boiling point. The boiling point of an azeotrope is either less than the temperature of constituents and is called a positive azeotrope.

Complete Step By Step Answer:
Given that Pressure is constant. So, this is an azeotropic mixture composition, because we can clearly make out that the line joining ${T_A}$ and ${T_B}$ is curved. It is positively deviated.
Which means it doesn’t follow Raoult’s law.
In the first option, it is wrong because the composition of azeotropic of distillate does not differ and it is the same from the mixture. So, we will eliminate this option.
In the second option it says that the boiling point goes on changing, but we are aware that in azeotropic compounds, the boiling point of a mixture doesn’t change. It has a constant boiling point mixture.
So, we will eliminate the second option too.
In the third option, the mixture has the highest vapour pressure at z point. So, the mixture doesn’t have the highest point. Hence, we will eliminate this option.
The (D) option is the composition will alter with change in normal pressure. This is the correct option because composition of azeotrope will alter with exchange in normal pressure.

Note :
Azeotropic distillation is usually a specific technique of adding another component to obtain a new compound with lower azeotrope. This is heterogeneous and has immiscible liquid phases. The best example is benzene to water and ethanol.