Question

Which of the following is not a protonic acid?
(A) $PO{{(OH)}_{3}}$
(B) $B{{(OH)}_{3}}$
(C) $SO{{(OH)}_{2}}$
(D) $S{{O}_{2}}{{(OH)}_{2}}$

Answer 1

Hint: It is a weak monobasic acid. The central atom of this acid belongs to the group thirteen. It is capable of accepting an electron pair and thus is a Lewis acid.

Complete step by step solution:
> A protonic acid is that acid which releases a proton in the aqueous solution. A hydrogen attached to a highly electronegative element is released as a proton in the aqueous solution.
> Protonic acids are explained on the basis of Lowry-Bronsted theory. According to the theory an acid is a substance which can release a proton (${{H}^{+}}$) in the aqueous solution. But, there are other theories also which explain acidic-basic behavior of substances.
> Lewis theory explains the acidic and basic behavior of substances on the basis of ability to accept or donate an electron pair. A substance which can accept an electron pair is a Lewis acid while a substance which can donate an electron pair is a Lewis base.
> Now, let’s analyze the given options.
- In option (A) $PO{{(OH)}_{3}}$ is given. It can be written as ${{H}_{3}}P{{O}_{4}}$. It is a triprotic acid. It contains three H attached to O. It is also called a tribasic acid. So, it is not the correct option.
- In option (C) $SO{{(OH)}_{2}}$ is given. It is a weak diprotic acid. It can be written as ${{H}_{2}}S{{O}_{3}}$. So it is not the correct option.
- In option (D) $S{{O}_{2}}{{(OH)}_{2}}$ is given. It is a strong diprotic acid. It can be written as ${{H}_{2}}S{{O}_{4}}$. So, it is not the correct option.
- In option (B) $B{{(OH)}_{3}}$ is given. It is weak Lewis acid. It does not furnish protons. It accepts an electron pair. So, it is the correct option.
Hence, the answer to the above question is option (B).

Note: $B{{(OH)}_{3}}$ acts as a Lewis acid as the central atom ‘B’ has unfilled p orbitals which are capable of accepting an electron pair.