Question

An azeotropic mixture of HCL and water has –
(A) 84% of HCI
(B) 22.2% HCI
(C) 63% of HCI
(D) 20.2% HCI

Answer 1

Hint: An azeotropic mixture of HCL and water is a negative boiling azeotrope. It can also be called a maximum boiling azeotrope. It has more concentration of HCl than water.

Complete step by step solution:
- Azeotropes are mixtures of two or more liquids that have the same concentration in both vapour phase and liquid phase.
- This means that when an azeotrope is boiled, the vapour of the constituents will have the same proportion of the constituents as the unboiled mixture.
- Because of this property of azeotropes, their composition will be unchanged upon simple distillation and hence this method cannot be used for separating the components.
- They are also called as constant boiling point mixtures as the mixture boils at a constant temperature.
- Azeotropes do not follow Raoult’s law (${{P}_{A}}={{P}_{A}}^{O}\times {{X}_{A}}$ ).
- Azeotropes are classified depending on the type of deviation from Raoult’s law.
- A solution showing negative deviation from Raoult’s law is called a maximum boiling azeotrope.
- A solution showing positive deviation from Raoult’s law is called a minimum boiling azeotrope.
- Azeotropes can be classified into positive and negative azeotropes on the basis of the characteristic boiling point of the azeotrope.
- The characteristic boiling point of an azeotrope will be either lesser or greater than the boiling point of any of its constituents.
- If the boiling point of the azeotropic mixture is lesser than the boiling point of any of its constituents, it is said to be a positive azeotrope.
- If the boiling point of the azeotropic mixture is greater than the boiling point of any of its constituents, it is said to be a negative azeotrope.
- From these 2 classifications, we can understand that positive azeotropes are minimum boiling azeotropes and negative azeotropes are maximum boiling azeotropes.
- The positive and negative azeotropes can be represented in a graph as-

- The azeotrope given in the question is a mixture of HCl and water. This is an example of a negative azeotrope.
- This azeotrope has a concentration of 20.2% hydrochloric acid and 79.8% water (by mass).
-Hence, the answer is option (D) 20.2% HCI.

Note: Azeotropes can also be classified on the basis of the miscibility of its components. A homogeneous azeotrope is one in which the constituents of the mixture are completely miscible in all proportions. A heterogeneous azeotrope is one in which the constituents of the mixture are not completely miscible.