Which of the following is not a Lewis base?

Which of the following is a conjugate acid-base pair?

$HNO_3$ in liquid hydrogen fluoride behaves as:

The correct order of increasing basicity of the given conjugate bases $(R = CH_3)$ is

The compound $HCl$ behaves as $...$ in the reaction,$HCl + HF \to H_2Cl^{+} + F^{-}$

$HF + HClO_4 \rightleftharpoons H_2F^{+} + ClO_4^-$ and $H_2O + H_2PO_4^- \rightleftharpoons H_3O^{+} + HPO_4^{2-}$ Select the group of acidic species present on the right side of the equilibrium.