Question

An azeotropic solution of two liquids has boiling point lower than that of either of them if it:
A) Shows a negative deviation from raoult's law
B) Shows a positive deviation from raoult's law
C) Shows no deviation from raoult's law
D) Is saturated

Answer 1

Hint:In order to solve this question we need to know how the raoult's law shows the positive and negative deviations. In case of positive deviation from raoult's law observed vapour pressure is greater than the ideal vapour pressure and boiling point of azeotrope becomes lower than either of pure liquid.

Complete answer:
For solving the questions related to raoult's law we need to know what is raoult's law:
So Raoult's law states that the vapour pressure of solvent above a solution is equal to vapour pressure of pure solvent at the same temperature scaled by the mole fraction of the solvent present.
Mathematically raoult's law equation is written as;
${P_{solvent}} = {X_{solvent}}{P^0}_{solvent}$
 Where,
${P_{solution}}$ = vapour pressure of the solution
${X_{solvent}}$ = mole fraction of the solvent
${P^0}_{solvent}$ = vapour pressure of pure solvent
When a solution shows positive deviation from Raoult's law, the force which is present in between the molecules or in simple intermolecular forces are weaker than which are present in the original solution. Then the total vapour pressure of the solution is greater than the total vapour pressure of the original solution. Also, it was found that the vapour of each component (solute and solvent) is greater than predicted from Raoult's law. Hence, we can say that the azeotropic mixture of two liquids boils at a lower temperature than either of them.
Hence (b) option is correct.

Note:We should know about Positive deviation and negative deviation of Raoult's law .Raoult's law positive deviation means the higher than expected vapour pressure when the non volatile solute is added to the solvent in simple terms we can say that solution. Conversely negative deviation means when the vapour pressure is lower than the expected pressure of the solution.
So here it will show positive deviation.