Question

A mixture of phenol and benzoic acid will completely dissolve in an aqueous solution of?
(A) HCl
(B) NaCl
(C) $NaHC{O_3}$
(D) NaOH

Answer 1

Hint: We know that phenol and benzoic acid are both acids and so they react with base to form soluble salts. But one of them is a strong acid while the other is only mildly acidic and the mildly acidic compound will react with only a strong base to form soluble salt but the strong acid can react with both strong and weak base. So, check the options for the suitable strong base.

Complete step by step answer:
-First of all we will see what phenol and benzoic acid is and about their nature.
Phenol is an aromatic compound having a molecular formula of ${C_6}{H_5}OH$. It is a mildly acidic compound due to presence of hydroxyl group (-OH) and is also volatile.
Benzoic acid is the simplest aromatic carboxylic acid with a molecular formula of ${C_6}{H_5}COOH$.
Among phenol and benzoic acid, benzoic acid is most acidic. Their structures are as follows:
               

-Coming back to the question, we know that an acid reacts with a base to form a salt and an acid will not react with another acid to form a salt or anything soluble in aqueous solution.
Since phenol and benzoic acid are both acidic in nature and benzoic acid is a strong acid, they can react with a strong base like NaOH to form their respective salts. The involved reactions are:
${C_6}{H_5} - COOH + NaOH \to {C_6}{H_5} - COONa + {H_2}O$
${C_6}{H_5} - OH + NaOH \to {C_6}{H_5}ONa + {H_2}O$

Also they cannot react with acids. Since benzoic acid is a strong acid it will react with both weak base (like $NaHC{O_3}$) and strong base (NaOH) to form salts soluble in aqueous solution, but phenol being a mild acidic compound cannot react with a weak base ($NaHC{O_3}$) to form any salt.

-Now let us take a look at the options:
For (A) HCl: Hydrochloric acid is a strong acid and so it cannot form salts with other acids.
For (B) NaCl: It is Sodium chloride and is a salt so it cannot form salt with other acids.
For (C) $NaHC{O_3}$: It is a weak base and so does not react with phenol.
For (D) NaOH: Sodium hydroxide is a strong base and so it reacts with both benzoic acid and phenol as discussed above to form salts.
So, the correct answer is “Option D”.

Note: Benzoic acid is a stronger acid than phenol because after losing ${H^ + }$ ion benzoic acid forms benzoate ion which is stabilized by two resonating structures where the negative charge is present on more electronegative oxygen atom while in the phenoxide ion formed by phenol carries negative charge on less electronegative carbon atom. Hence benzoate ion is more stable than phenoxide ion and benzoic acid is stronger acid than phenol.