Three reactions involving `H_(2)PO_(4)^(-)` are given below
`I. H_(3)PO_(4)+H_(2)OrarrH_(3)O^(+)+H_(2)PO_(4)^(-)`
`II. H_(2)PO_(4)^(-)+H_(2)OrarrHPO_(4)^(2-)+H_(3)O^(+)`
`III. H_(2)PO_(4)^(-)+OH^(-)rarrH_(3)PO_(4)+O^(2+)`
In which of the above does `H_(2)PO_(4)^(-)` act as an acid?

Options

A. (ii) only
B. (i) and (ii)
C. (iii) only
D. (i) only

Answer

A

Explanation

(i) `{:(H_(3)PO_(4)+H_(2)OhArrH_(3)O^(+)+,H_(2)PO_(4)^(-)),("acid","conjugate base"):}`
In reverse reaction `H_(2)PO_(4)^(-)` is accepting a proton, so it base.
(ii) `{:(H_(2)PO_(4)^(-)+H_(2)OhArr,HPO_(4)^(2-),+H_(3)O^(+)),("acid","conjugate base",):}`
`H_(2)PO_(4)^(-)` is donating a proton, which is accepting by `H_(2)O`, so it is an acid.
(iii) `{:(H_(2)PO_(4)^(-)+,OH^(-)hArr,H_(3)PO_(4),+O^(2-)),("acid","acid",conjugate base",):}`
`H_(2)PO_(4)^(-)` is a accepting a proton, so it is base. Only in reaction (ii) `H_(2)PO_(4)^(-)` acids as an acid.