Which of the following substances is insoluble in acetic acid?

Which of the following happens when $NH_{4}OH$ is added gradually to the solution containing $1 \ M \ A^{2+}$ and $1 \ M \ B^{3+}$ ions?
Given: $K_{sp}[A(OH)_{2}] = 9 \times 10^{-10}$ and $K_{sp}[B(OH)_{3}] = 27 \times 10^{-18}$ at $298 \ K$.

The solubility product of $PbS$ is $3.4 \times 10^{-28}$. If $[Pb^{2+}] = 1 \times 10^{-2} \ mol/L$,what concentration of $[S^{2-}]$ is required to precipitate $PbS$?

What is the concentration of $Ba^{2+}$ when $BaF_2$ $(K_{sp} = 1.0 \times 10^{-6})$ begins to precipitate from a solution that is $0.30 \, M \ F^{-}$?

The molar solubility of $Zn(OH)_{2}$ in $0.1 \, M \, NaOH$ solution is $x \times 10^{-18} \, M$. The value of $x$ is ...... . (Nearest integer)
(Given : The solubility product of $Zn(OH)_{2}$ is $2 \times 10^{-20}$)

The salt having the least solubility in water is