When $H_2S$ gas is passed through the $HCl$ containing aqueous solutions of $CuCl_2$,$HgCl_2$,$BiCl_3$ and $CoCl_2$,which does not precipitate out?

Which one among the following pairs of ions cannot be separated by $H_2S$ in dilute hydrochloric acid?

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

$MgSO_4$ on reaction with $NH_4OH$ and $Na_2HPO_4$ forms a white crystalline precipitate. What is its formula?

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

To a solution of a substance,gradual addition of ammonium hydroxide results in a black precipitate which does not dissolve in excess of $NH_4OH$. However,when $HCl$ is added to the original solution,a white precipitate is formed. The solution contained