Passing $H_2S$ gas into a mixture of $Mn^{2+}, Ni^{2+}, Cu^{2+}$ and $Hg^{2+}$ ions in an acidified aqueous solution precipitates

$A$ reagent used to test the presence of $Fe^{2+}$ ion is

Consider the following sequence of reactions:
$M^{2+}(aq.)$ $\xrightarrow{NH_4Cl(s) + (NH_4)_2CO_3 \, sol.} Q \downarrow$ $\xrightarrow[\text{Followed by addition of } (NH_4)_2C_2O_4]{CH_3COOH} R \downarrow$
Which of the following cations can form precipitate $Q$ but does not form precipitate $R$?

Which ion$(s)$ precipitate when an oxalate solution is added to a mixture of $Ba^{2+}$,$Sr^{2+}$,and $Ca^{2+}$?

Salt $X$ is:
$X \text{ (Colourless salt)}$ $\xrightarrow[\Delta]{NaOH} Y$ $\xrightarrow{HCl} \text{White fumes}$
$X \text{ (Colourless salt)}$ $\xrightarrow{CaCl_2} Z \text{ (White ppts.)}$ $\xrightarrow{KMnO_4} \text{Colourless solution}$

The colour of $Mg(OH)_2$ precipitate formed when it is precipitated in the presence of magneson reagent is