The conjugate base of $H_2PO_4^-$ is

The conjugate base of the strong acid in the reaction $CH_3COOH + HCl \rightleftharpoons Cl^{\Theta} + CH_3COOH_2^{\oplus}$ is:

For the following two acid-base reactions,which statement is True?
$(I) CH_3CH_2^- + CH_3NH_2$ $(pK_a = 35)$ $\rightleftharpoons CH_3CH_3$ $(pK_a = 50)$ $+ CH_3NH^-$
$(II) F^- + H_2O$ $(pK_a = 15.7)$ $\rightleftharpoons HF$ $(pK_a = 3.2)$ $+ HO^-$

$A$ tribasic acid is:

Three reactions involving $H_2PO_4^-$ are given below :
$(i) \; H_3PO_4 + H_2O \rightarrow H_3O^{+} + H_2PO_4^-$
$(ii) \; H_2PO_4^- + H_2O \rightarrow HPO_4^{2-} + H_3O^{+}$
$(iii) \; H_2PO_4^- + OH^{-} \rightarrow H_2O + HPO_4^{2-}$
In which of the above does $H_2PO_4^-$ act as an acid?

The conjugate base of $H_{2}PO_{4}^{-}$ is: