Which one of the following substances has the highest proton affinity?

$HNO_3$ in liquid hydrogen fluoride behaves as:

The dissociation constants of two acids $HA_1$ and $HA_2$ are $3.0 \times 10^{-4}$ and $1.8 \times 10^{-5}$ respectively. The relative strengths of the acids will be

$HNO_3$ in aqueous solution yields:

The conjugate base of $HPO_4^{2-}$ is:

Which of the following is the strongest conjugate base?