Question

Which one of the following species acts as both Bronsted acid and base?

• A. ${\text{H}}_2\text{P}{\text{O}}_2^{-}$
• B. $\text{H}\text{P}{\text{O}}_3^{-2}$
• C. $\text{H}\text{P}{\text{O}}_4^{-2}$
• D. All of these

Answer 1

Answer: (C)

$\text{H}\text{P}{\text{O}}_4^{2-}+{\text{H}}_2\text{O}\rightleftharpoons {\text{H}}_2\text{P}{\text{O}}_4^{-}+\text{O}{\text{H}}^{-}$

$\text{H}\text{P}{\text{O}}_4^{2-}+{\text{H}}_2\text{O}\rightleftharpoons \text{P}{\text{O}}_4^{3-}+{\text{H}}_3{\text{O}}^{+}$

${\text{H}}_2\text{P}{\text{O}}_2^{-}$ is a conjugate base of ${\text{H}}_3\text{P}{\text{O}}_2$ (a monobasic acid) and does not give ${\text{H}}^{+},\text{H}\text{P}{O}_3^{2-}$ is a conjugate base of ${\text{H}}_2\text{P}{\text{O}}_3^{-}$ and does not ionise further. Since, ${\text{H}}_2\text{P}{\text{O}}_3^{}$ is a dibasic acid, $\text{H}\text{P}{\text{O}}_4^{2-}$ can both accept or donate a proton.