The colour of the precipitate produced by adding $NaOH$ solution to $HgCl_2$ is

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

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

Identify ions $A$ and $B$,respectively,based on the following reaction scheme:

Precipitate of group $IV$ cations takes place when $H_2S$ is

To an aqueous solution containing ions such as $Al^{3+}, Zn^{2+}, Ca^{2+}, Fe^{3+}, Ni^{2+}, Ba^{2+}$ and $Cu^{2+}$,concentrated $HCl$ was added,followed by $H_{2}S$. The total number of cations precipitated during this reaction is/are: