Question

An aqueous solution of boric acid is found to be weakly acidic in nature. This acidic character arises due to the following reasons.
A. It is a protic acid which donates protons in aqueous solution.
B. It is a Lewis acid which abstracts $O{{H}^{-}}$ from water and leaves ${{H}^{+}}$ to make the solution acidic.
C. It gives metaboric acid when dissolved in water.
D. It is prepared by reaction of borax with sulphuric acid hence it behaves as an acid.

Answer 1

We know that a solution is said to be acidic when the concentration of hydrogen ions is greater than the hydroxide ions. So, look into each option and try to figure out which option is the suitable reason for the question.

Complete step by step answer:
Boron belongs to p-block and is the first member of its group. The formula of boric acid is ${{H}_{3}}B{{O}_{3}}$ and it is also known by the name “orthoboric acid”. It is a monobasic Lewis acid, where on reaction oxygen gives its one electron to boron and makes it unstable and further to become stable, boric acid furnishes hydrogen ion.
As, boron is small in size and boron has five electrons in its valence shell. So, boric acid, when it reacts with water, it accepts $O{{H}^{-}}$ions from water molecule and loses its proton that is hydrogen ions (${{H}^{+}}$). The following reaction is shown below,
${{H}_{3}}B{{O}_{3}}+{{H}_{2}}O\to {{H}_{3}}{{O}^{+}}+{{H}_{2}}B{{O}_{3}}^{-}$

So, here we can see that hydronium ions are released to the water solution and boric acid is abstracting hydroxyl ions. We know that when there is high concentration of hydronium ions, the solution is referred to as acidic in nature.
Thus, the second option suits the reason.
So, the correct answer is “Option B”.

Note: The options given in the question, many are correct but the main reason for the acidic character is due to release of hydrogen ions into the water and gaining electrons from the oxygen atom. So, you should not be confused with other options rather find the actual reason behind the acidic character of boric acid.