Three reactions involving ${\mathrm{H}}_2{{\mathrm{PO}}^{-}}_4$ are given below

(P) ${\mathrm{H}}_3{\mathrm{PO}}_4+{\mathrm{H}}_2\mathrm{O}⟶{\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{H}}_2{\mathrm{PO}}_4^{-}$

(Q) ${\mathrm{H}}_2{\mathrm{PO}}_4^{-}+{\mathrm{H}}_2\mathrm{O}⟶{\mathrm{HPO}}_4^{2-}+{\mathrm{H}}_3{\mathrm{O}}^{+}$

(R) ${\mathrm{H}}_2{\mathrm{PO}}_4^{-}+{\mathrm{OH}}^{-}⟶{\mathrm{H}}_3{\mathrm{PO}}_4+{\mathrm{O}}^{2-}$

In which of the above ${\mathrm{H}}_2{{\mathrm{PO}}^{-}}_4$ acts as an acid?

${\mathrm{H}}_2\mathrm{PO}{4}^{-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{HPO}}^{2-}+{\mathrm{H}}_3{\mathrm{O}}^{+}$

${\mathrm{H}}_2{{\mathrm{PO}}^{-}}_4$ behaves as an acid in the second reaction because it produces ${\mathrm{H}}^{+}$ in aqueous media.

${\mathrm{H}}_2{\mathrm{PO}}^{-}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{HPO}}^{2-}\left(\mathrm{aq}\right)$

${\mathrm{H}}_3{\mathrm{PO}}_4+{\mathrm{H}}_2\mathrm{O}⟶{\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{H}}_2{\mathrm{PO}}_4^{-}$

${\mathrm{H}}_3{\mathrm{PO}}_4$ is conjugated base, ${\mathrm{H}}_2{{\mathrm{PO}}^{-}}_4$, is a compound.

${\mathrm{H}}_2{\mathrm{PO}}_4^{-}+{\mathrm{H}}_2\mathrm{O}⟶{\mathrm{HPO}}_4^{2-}+{\mathrm{H}}_3{\mathrm{O}}^{+}$

${\mathrm{H}}_2{{\mathrm{PO}}^{-}}_4$ serves as a conjugate base in this reaction, donating one hydrogen to the parent acid to make it an acid.