Three reactions involving ${\text{H}}_2{\text{PO}}_4^{-}$ are given below:(i) ${\text{H}}_3{\text{PO}}_4+{\text{H}}_2\text{O}\to {\text{H}}_3{\text{O}}^{+}+{\text{H}}_2{\text{PO}}_4^{-}$ (ii) ${\text{H}}_2{\text{PO}}_4^{-}+{\text{H}}_2\text{O}\to {\text{HPO}}_4^{2-}+{\text{H}}_3{\text{O}}^{+}$ (iii) ${\text{H}}_2{\text{PO}}_4^{-}+{\text{OH}}^{-}\to {\text{H}}_3{\text{PO}}_4+{\text{O}}^{2-}$ (Inconsistent stoichiometry)In which of the above does ${\text{H}}_2{\text{PO}}_4^{-}$ act as an acid?

An acid is a proton donor.In (i), ${\text{H}}_2{\text{PO}}_4^{-}$ is the conjugate base of ${\text{H}}_3{\text{PO}}_4$.In (ii), ${\text{H}}_2{\text{PO}}_4^{-}$ donates a proton to ${\text{H}}_2\text{O}$ to form ${\text{HPO}}_4^{2-}$. Thus, it acts as an acid.In (iii), ${\text{H}}_2{\text{PO}}_4^{-}$ accepts a proton (incorrectly represented in the prompt) or reacts with base. Based on standard Bronsted-Lowry, only (ii) shows the species donating a proton.