The conjugate base of hydrazoic acid $(N_3H)$ is .......

In the equilibrium $CH_3COOH + HF \rightleftharpoons CH_3COOH_2^+ + F^{-}$,which of the following statements is correct?

Which of the following is not a Lewis base?

Which one of the following is not an amphoteric substance?

Which one of the following compounds is not a protonic acid?

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?