Addition of $SnCl_2$ to $HgCl_2$ gives a precipitate of which color?

On passing $H_2S$ gas,a black precipitate of $II$ group is obtained. The mixture may not contain:

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

Which of the following does not give a precipitate with $CrO_4^{2-}$?

In a solution containing $Hg^{2+}, Cd^{2+}, Sr^{2+}, Fe^{2+},$ and $Cu^{2+}$ ions in $0.3 \, M \, HCl$,which ions will precipitate upon the addition of $H_2S$?

$A$ solution containing a group $IV$ cation gives a precipitate on passing $H_2S$ gas. $A$ solution of this precipitate in dilute $HCl$ produces a white precipitate with $NaOH$ solution and a bluish-white precipitate with basic potassium ferrocyanide. The cation is: