Question

Which of the following is a conjugate acid-base pair?

• A. $H_{3}P{O}_4$ and $H_{2}P{O}_4^{-}$
• B. $H_{3}P{O}_3$ and $HP{O}_3^{-2}$
• C. $H_{2}P{O}_4^{-}$ and $P{O}_4^{-3}$
• D. $H_{2}P{O}_3^{-}$ and $P{O}_3^{-3}$

Answer 1

The correct option is A $H_{3}P{O}_4$ and $H_{2}P{O}_4^{-}$

A conjugate acid-base pair is the one in which the acid and its conjugate base or vice versa, differ in only one proton $\left(H^{+}\right)$

From the given options, there is only one such pair which is $H_{3}P{O}_4$ and $H_{2}P{O}_4^{-}$.

Now, $H_{3}P{O}_4⟶H^{+}+H_{2}P{O}_4^{-}$

Here, the acid $H_{3}P{O}_4$ loses a proton to give $H_{2}P{O}_4^{-}$ which is its conjugate base.

Hence $H_{3}P{O}_4$ and $H_{2}P{O}_4^{-}$ is a conjugate acid-base pair.