Which of the following is most soluble in water?

$H_{2}S$ is passed into $1 \ L$ of a solution containing $0.1 \ mole$ of $Zn^{2+}$ and $0.01 \ mole$ of $Cu^{2+}$ until the sulphide ion concentration reaches $8.1 \times 10^{-19} \ M$. Which one of the following statements is true? $[K_{sp}$ of $ZnS$ and $CuS$ are $3 \times 10^{-22}$ and $8 \times 10^{-36}$ respectively.$]$

Solubility product of $CaC_2O_4$ at a given temperature in pure water is $4 \times 10^{-9} \ (mol \ L^{-1})^2$. Solubility of $CaC_2O_4$ at the same temperature is

If the solubility product of calcium fluoride $(CaF_2)$ is $3.2 \times 10^{-11}$,then its solubility will be ..........

In the reaction $Fe(OH)_{3(s)} \rightleftharpoons Fe^{3+}_{(aq)} + 3OH^{-}_{(aq)}$, if the concentration of $OH^{-}$ ions is decreased by $\frac{1}{4}$ times, then the equilibrium concentration of $Fe^{3+}$ will increase by: (in $times$)

The $K_{sp}$ value for $Gd(OH)_3$ is $2.7 \times 10^{-23}$. At what $pH$ will $Gd(OH)_3$ begin to precipitate? (Given: $\log 3 = 0.48$)