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

A monobasic weak acid dissociates to $1.2 \%$ in its 0.01 M solution at 298 K . Calculate dissociation constant of it.

The solubility of calcium carbonate at 298 K is $6.4 \times 10^{-5} \mathrm{~mol} \mathrm{dm}^{-3}$. Calculate the value of solubility product at the same temperature?

The solubility product of $\mathrm{PbI}_2$ is $1.08 \times 10^{-7}$.

Calculate its solubility in $\mathrm{mol} \mathrm{dm}^{-3}$ at 298 K .