Question

The correct statement(s) for orthoboric acid is(are)

• A. It behaves as a weak acid in water due to self ionization
• B. Acidity of its aqueous solution increases upon addition of ethylene glycol
• C. It has a three-dimensional structure due to hydrogen bonding
• D. It is a weak electrolyte

Answer 1

Answer: (B), (D)

The correct options are
B

Acidity of its aqueous solution increases upon addition of ethylene glycol

D

It is a weak electrolyte

$H_{3}B{O}_3+H_{2}O\rightleftharpoons \left[B\right(OH{)}_4{]}^{-}+H^{+}$

Clearly, self-ionization is not in play here. The pK_a value is 9.25 and this clearly shows that boric acid is a weak electrolyte. Addition of a diol like ethylene glycol and other cis-diols consume the species $\left[B\right(OH{)}_4{]}^{-}$ to drive the above reaction in the forward reaction. When the reaction moves forward, more of $H^{+}$ is formed and thus the acidity of the system increase.

$H_{3}B{O}_3$ has a sheet like structure owing to hydrogen bonding. It is not three-dimensional.