Question

Which species acts as an acid and also a conjugate base of another acid?

• A. $H{SO}_4^{-}$
• B. ${CO}_3^{2-}$
• C. ${SO}_4^{2-}$
• D. $H_3{O}^{+}$

Answer 1

Answer: (A)

$H{SO}_4^{-}$

The concept of conjugate acid-base pair was given by Lowry-Bronsted Theory.
According to the theory:
Acids are proton $\left(H^{+}\right)$ donors
Bases are proton $\left(H^{+}\right)$ acceptors
Hence, in order to get the conjugate base of a species, we should consider that substance as an acid and vice versa.
For example,
$O{H}^{-}\to O^{-2}+H^{+}$
$O{H}^{-}+H^{+}\to H_{2}O$
Hence,
$O{H}^{-}$ act as both acid as well as base.
Using this logic, for the given example:
$HS{O}_4^{-}+H^{+}\to H_{2}S{O}_4$
$HS{O}_4^{-}\to S{O}_4^{-2}+H^{+}$
Therefore,
$HS{O}_4^{-}$ acts as both acid as well as base.
In other options,
$C{O}_3^{-2}$ can accept but not donate a proton.
$S{O}_4^{-2}$ can accept but not donate a proton.
$H_3{O}^{+}$ can donate but not accept a proton.