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.