Question

Which of the following can be called as a conjugate acid - base pair?

• A. $HCl,NaOH$
• B. $H_3{O}^{+}/O{H}^{-}$
• C. $H_{2}S{O}_4,HS{O}_4^{-}$
• D. $H_{2}S/S^{2-}$

Answer 1

The correct option is C $H_{2}S{O}_4,HS{O}_4^{-}$

Conjugate acid: Species formed by the gain of a proton from a base is called conjugate acid of that base. [Conjugate acid has one extra proton than the base]
Conjugate base : Species formed by the loss of a proton from an acid is called the conjugate base of that acid.
[Conjugate base has one proton less than the acid]
So, Conjugate acid-base pairs differ only by one proton.

$H^{+}+HS{O}_4^{-}\rightleftharpoons H_{2}S{O}_4$
Here $HS{O}_4^{-}$, (base) gains a proton $H^{+}$ to form $H_{2}S{O}_4$ (conjugate acid).


$H_{2}O\left(I\right)\stackrel{H^{+}}{\to }{H}_3{O}^{+}\left(aq\right)$.So
$H_{2}O\left(I\right)$ is base and $H_3{O}^{+}\left(aq\right)$ is conjugate acid. Therefore , $H_3{O}^{+}andO{H}^{-}$ is not a conjugate acid - base pair.


$S^{2-}\stackrel{H^{+}}{\to }H{S}^{-}$.So
$S^{2-}$ is base and $H{S}^{-}$ is conjugate acid. So , $H_{2}Sand{S}^{2-}$ is not a conjugate acid - base pair.