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

Among the following cations,the number of cations which will give characteristic precipitate in their identification tests with $K_4[Fe(CN)_6]$ is :
$Cu^{2+}, Fe^{3+}, Ba^{2+}, Ca^{2+}, NH_4^{+}, Mg^{2+}, Zn^{2+}$

The reagent that can distinguish between silver and lead salt is

Consider the following reactions:
$X(aq.) \xrightarrow{K_4[Fe(CN)_6]}$ Chocolate brown ppt.
$X(aq.) \xrightarrow{AgNO_3}$ White ppt. (insoluble in dil. $HNO_3$)
Then $X$ will be:

Which of the following basic radicals will not be precipitated by $H_2S$ gas in the presence of $NH_3$?

Lead sulphate is soluble in: