Which of the following is least soluble in water?

The $pH$ of a saturated solution of $Ca(OH)_2$ is $12.25$. Calculate its solubility product $(K_{sp})$.

If the concentration of $[Ag^{+}]$ in a solution is $1 \times 10^{-6} \ M$,what is the concentration of $[Br^{-}]$? Given that the $K_{sp}$ for $AgBr$ is $4.0 \times 10^{-13}$.

Which of the following substances is insoluble in dilute $HCl$?

Solubility product constants $(K_{sp})$ of silver salts are given as $AgCl = 2 \times 10^{-10}$,$AgBr = 5 \times 10^{-13}$,$Ag_2CO_3 = 8 \times 10^{-12}$,and $AgI = 8 \times 10^{-17}$. Which salt shows the highest solubility?

Sodium sulfate is soluble in water,whereas barium sulfate is sparingly soluble. Why?