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.