Question

Assertion: A solution of phenol and aniline will show negative deviations from Raoult's law.
Reason: In case of negative deviations from Raoult's law, A-B forces are stronger than A-A and B-B forces
A. Both assertion and reason are correct and reason is correct explanation for assertion.
B. Both assertion and reason are correct and reason is not the correct explanation for assertion.
C. Assertion is correct but reason is incorrect.
D. Assertion as well as reason are both incorrect.

Answer 1

Hint: Negative deviations from Raoult’s Law are shown by binary mixtures whose vapour pressure is lesser than what is predicted by Raoult’s Law. Deviations from this law are shown when there is a force of attraction or repulsion between the two components. If the interaction between the two components is stronger than forces in either of the pure solvents, negative deviation is seen.

Complete step by step solution:
-Raoult’s Law is a law which says that the partial pressure exerted by a component in a binary mixture is the product of its mole fraction in solution and its vapour pressure as a pure solvent.
Let us take a look at both the statements.
-A solution of phenol and aniline, when mixed together will have a lower vapour pressure than that of the vapour pressure of pure components. This is because of the interaction between aniline and phenol molecules through hydrogen bonding. Since there exists a force of attraction between the molecules in the liquid phase, a lesser amount of both the components will be present in the vapour phase and thus, this solution shows negative deviation from Raoult’s Law. Hence, the assertion is true.
-The reason statement says that in case of negative deviations from Raoult's law, A-B forces are stronger than A-A and B-B forces, which is exactly why the assertion statement is true, as we have seen from above. The A-B forces mentioned here are nothing but the hydrogen bonds.
-Hence, from our observations, both assertion and reason are correct and reason is the correct explanation for assertion.
Therefore, the correct option to be marked is option A.

Note:
When the two components attract each other in the liquid phase, the A-A forces (forces between similar molecules) are weaker than the A-B forces (forces between different molecules) and hence, the solution will show negative deviation. Note that the solutions which obey Raoult's Law over a wide range of concentrations are known as ideal solutions.