The salt having the least solubility in water is

The solubility product of salt $B_2A$ is $3.2 \times 10^{-11}$ at $298 \ K$. What is the solubility of the salt at the same temperature?

What is the minimum volume of water required to dissolve $1 \, g$ of calcium sulphate at $298 \, K$? (For calcium sulphate,$K_{sp}$ is $9.1 \times 10^{-6}$).

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 $4.0 \times 10^{-13}$.

Assign $A$,$B$,$C$,$D$ from the given type of reaction.
$HgS \downarrow + HNO_3 \text{ (Conc.)} \longrightarrow \text{No dissolution}$

The solubility product of $Mg(OH)_2$ is $1.0 \times 10^{-12}$. Concentrated aqueous $NaOH$ solution is added to a $0.01 \ M$ aqueous solution of $MgCl_2$. The $pH$ at which precipitation occurs is