The common ion effect of ${\mathrm{NH}}_4^{+}$ ions causes the precipitation group$-\mathrm{III}$ cationic radicals

Give reasons for the following :

A mixture of ${\mathrm{NH}}_4\mathrm{CI} \mathrm{and} {\mathrm{NH}}_4\mathrm{OH}$ is used to precipitate the metallic hydroxides of group III.

Represent the union of two sets by Venn diagram for each of the following.

$X=\{x | x$ is a prime number between $80$ and $100\}$

$Y=\{y | y$ is an odd number between $90$ and $100\}$

Explain the principle involved in the following observations.

${\mathrm{Cu}}^{2+}$is precipitated as $\mathrm{CuS}$ in acidic medium while ${\mathrm{Ni}}^{2+}$ is precipitated as $\mathrm{NiS}$ in basic medium with ${\mathrm{H}}_2\mathrm{S}$.