Question

Boric acid ($B{{(OH)}_{3}}$) is:
This question has multiple correct options
(A)- Tribasic
(B)- Dibasic
(C)- Monobasic
(D)- Aprotic

Answer 1

Hint: Boric acid (${{H}_{3}}B{{O}_{3}}$or$B{{(OH)}_{3}}$) also goes by the name orthoboric acid. It is a very weak Lewis acid. Lewis acids are generally electron deficient compounds. For example,$AlC{{l}_{3}}$and$B{{F}_{3}}$. etc. Aprotic acids do not have any acidic hydrogen. The word aprotic is broken as a-protic, a means without and protic means proton.
Point to remember: $Acidity\propto {{K}_{a}}\propto \frac{1}{p{{K}_{a}}}$
where, ${{K}_{a}}$ is acid dissociation constant and $p{{K}_{a}}=-\log {{K}_{a}}$.

Complete step by step solution:
The first dissociation constant of boric acid is ${{K}_{a}}=5.8\times {{10}^{-10}}$which is quite low and hence, it behaves as a very weak acid. It is an aprotic acid i.e. it does not release protons. Electronic configuration of boron in ${{H}_{3}}B{{O}_{3}}$ reveals that it has only six electrons in its valence shell. Therefore, it has an ability to accept a pair of electrons to attain stable noble gas configuration. It accepts a pair of electrons from water (in the form of $O{{H}^{-}}$ ion) to release a proton and thus acts as a Lewis acid.
$B{{(OH)}_{3}}+H-OH\to {{\left[ B{{(OH)}_{4}} \right]}^{ }}+{{H}^{+}};p{{K}_{a}}=9.25$
We can, thus, say that boric acid is not a proton donor but an electron pair acceptor.
$B{{(OH)}_{3}}$ can accept only one hydride ion. It is because boron being smaller in size cannot accommodate more ligands around it. Consequently, it is a monobasic acid.
Considering both the arguments, we have reached to a conclusion that boric acid is a monobasic aprotic acid. Hence, the correct options are option (C) and option (D).

Additional Information: Boric acid is crystalline in nature and has a soapy touch. It is generally prepared by heating borax ($N{{a}_{2}}{{B}_{4}}{{O}_{7}}$) with $HCl\text{ or }{{H}_{2}}S{{O}_{4}}$ and also by hydrolyzing boron compounds.

Note: We may mistake boric acid as a tribasic acid because of the three $O{{H}^{-}}$ ions in its molecular formula. ${{H}_{3}}B{{O}_{3}}$ can also be written as $B{{(OH)}_{3}}$ which is mostly likely to cause such confusion.