Question

Assertion: $p{K_a}$ of acetic acid is lower than that of phenol.
Reason: Phenoxide ion is more resonance stabilised.
(A) Both assertion and reason are correct and reason is the correct explanation for assertion.
(B) Both assertion and reason are correct but reason is not the correct explanation for assertion.
(C) Assertion is correct but reason is incorrect.
(D) Both assertion and reason are incorrect.

Answer 1

For more acidic the compound the value of $p{K_a}$ will be less. Here we know that acetic acid is more acidic than phenol so the value of $p{K_a}$ for acetic acid is less than the value of $p{K_a}$ of phenol.

Complete step by step solution:
First of all let us understand the acid and basic nature of the compound.
Acid nature: The compound which releases hydrogen ions on hydration. For example: carboxylic acid. Carboxylic acid on hydration gives hydrogen ions.
Basic nature: The compound which releases hydroxide ion on hydration. For example: alcohol. Alcohol on hydration gives hydroxide ion.
Now the value of $p{K_a}$ for a more acidic compound will be less and will be more for a less acidic compound.
Here we are given with the compounds acetic acid and phenol. And we know that the acetic acid is more acidic than phenol. So the $p{K_a}$ value for acetic acid is less than that of phenol. This means that carboxylate ions should be more stable than phenoxide ions. Stability of ions can be understood by the number of resonating structures formed by ions. Here the carboxylate ion has only two resonating structures. And the phenoxide ion has six resonating structures. So $p{K_a}$ of acetic acid is lower than that of phenol. And the reason is because the phenoxide ion is more stabilised due to resonance. So assertion and reason are correct but reason is not the correct explanation of assertion. Hence option b is correct.

Note:
$p{K_a}$ value is defined as the value to measure the strength of the acid. It is defined as the negative logarithm of the acid dissociation constant or ${K_a}$ value. Lower value indicates the acid is more dissociable in water.