Question

Boric acid $B(OH{)}_3$ is/are:

• A. Tribasic
• B. Dibasic
• C. Monobasic
• D. Aprotic

Answer 1

Answer: (C), (D)

Aprotic

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.
$B(OH{)}_3+H-OH\to [B\left(OH{)}_4\right]+H^{+};p{K}_a=9.25$
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 conclusing that boric acid is a monobasic aprotic acid. Hence, the correct options are option (C) and option (D).