What are the products formed when an aqueous solution of $CuCl_2$ and $(NH_4)_2S$ are mixed?

Solid $Ba(NO_3)_2$ is gradually dissolved in $1.0 \times 10^{-4} \, M \, Na_2CO_3$ solution. At which concentration of $Ba^{2+}$ will the precipitation begin? $(K_{sp} \, BaCO_3 = 5.1 \times 10^{-9})$

What is the solubility of a salt? Explain the classification of salts based on solubility.

Assign $A, B, C, D$ from the given type of reaction.
$Ca(OH)_2 + 2HF \longrightarrow CaF_2 \downarrow + 2H_2O$

At $T(K)$ in a saturated solution of $MgCO_3$ and $Ag_2CO_3$, if the concentration of $Mg^{2+}$ ion is $3.2 \times 10^{-5} \ M$, then the concentration of $Ag^{+}$ ion in the solution will be [Given: $K_{sp}(MgCO_3) = 1.6 \times 10^{-6}$ and $K_{sp}(Ag_2CO_3) = 8.0 \times 10^{-12}$ at $T(K)$]

The solubility of $PbF_2$ in water at $25\,^{\circ}C$ is $\sim 10^{-3}\, M$. What is its solubility in $0.05\, M\, NaF$ solution? Assume the latter to be fully ionised.