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

Options

A. `H_(2)PO_(2)^(-)`
B. `HPO_(3)^(2-)`
C. `HPO_(4)^(2-)`
D. All of these

Answer

C

Explanation

`HPP_(4)^(2-)+H_(2)O hArr H_(2) PO_(4)^(2-)+OH^(-)`
`HPO_(4)^(2)- hArr H_(2)O hArr PO_(4)^(3-)+H_(3)O^(+)`
`H_(2)PO_(2)^(-)` is a conjugate bse of `H_(3)PO_(2)` (a monobasic acid) and does not give `H^(+), HPO_(3)^(2-)` is a conjugate base of `H_(2)PIO_(3)^(-)` and does not ionis further, since `H_(3)PO_(3)` s a dibasic acid.