Identify the conjugate acid-base pair respectively from following equilibrium reaction.

$$ \mathrm{HPO}_{4(\mathrm{aq})}^{2-}+\mathrm{H}_2 \mathrm{O}_{(\ell)} \rightleftharpoons \mathrm{PO}_{4(\mathrm{aq})}^{3-}+\mathrm{H}_3 \mathrm{O}_{(\mathrm{eq})}^{+} $$

The solubility of sparingly soluble salt $\mathrm{AX}_2$ is $1 \times 10^{-4} \mathrm{~mol} \mathrm{dm}^{-3}$ at 298 K . Calculate its solubility product.

Calculate the value of dissociation constant of weak monoacidic base if it dissociates to $2 \%$ in 0.1 M solution?

Which from following formulae is used to find the $\left[\mathrm{OH}^{-}\right]$ion concentration of a weak monoacidic base?