Question

The conjugate base of $H_3{BO}_3$ is

• A. $B{\left(OH\right)}_4^{-}$
• B. $H_3{BO}_3^{-}$
• C. $H{BO}_3^{2-}$
• D. $H_4{BO}_3^{+}$

Answer 1

The correct option is A $B{\left(OH\right)}_4^{-}$

The concept of Conjugate Acid-Base pair was given by Lowry-Bronsted Theory.
According to the theory, acids are proton $\left(H^{+}\right)$ donors or acceptors of $O{H}^{-}$ and bases are proton $\left(H^{+}\right)$ acceptors or donors of $O{H}^{-}$.
Hence, in order to get the conjugate base of a species, we should consider that substance as an acid and vice versa.
For example,
$H_{2}S{O}_4\to HS{O}_4^{-}+H^{+}$

Therefore,
Conjugate base of $H_{2}S{O}_4$ is $HS{O}_4^{-}$.
Using this logic, for the given example:
$H_{3}B{O}_3\to H_{2}B{O}_3^{-}+H^{+}$

Hence,
Conjugate base of $H_{3}B{O}_3$ is $H_{2}B{O}_3^{-}$.
This does not match with any of the options.
Hence, we use the second definition: Acids accept $O{H}^{-}$

Therefore,
$H_{3}B{O}_3+H_{2}O\to B(OH{)}_4^{-}+H^{+}$
So, $B(OH{)}_4^{-}$ is the conjugate base of $H_{3}B{O}_3$.