Conjugate acid of $HPO_4^{2-}$ is

Which of the following is the weakest Bronsted-Lowry base?

The first and second dissociation constants of an acid $H_2A$ are $1.0 \times 10^{-5}$ and $5.0 \times 10^{-10}$ respectively. The overall dissociation constant of the acid will be

Which of the following is not a Lewis base?

Write the conjugate base of the following: $HF, CH_{3}NH_{3}^{+}, H_{3}PO_{4}, HPO_{4}^{2-}$

If the $pK_b$ of the fluoride ion $(F^-)$ at $25^{\circ}C$ is $10$,what is the ionization constant $(K_a)$ of hydrofluoric acid $(HF)$ at the same temperature?