Which compound is used to distinguish between $Fe^{2+}$ and $Fe^{3+}$ ions?

An inorganic compound $X$ on treatment with concentrated $H_{2}SO_{4}$ produces brown fumes and gives a dark brown ring with $FeSO_{4}$ in the presence of concentrated $H_{2}SO_{4}$. Also,compound $X$ gives a precipitate $Y$ when its solution in dilute $HCl$ is treated with $H_{2}S$ gas. The precipitate $Y$ on treatment with concentrated $HNO_{3}$ followed by excess of $NH_{4}OH$ further gives a deep blue coloured solution. Compound $X$ is:

What precipitate is formed when $SnCl_2$ is added to $HgCl_2$?

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

Which of the following reagents is used to detect the presence of $Fe^{3+}$ ions in a solution?

Aqueous solution of which of the following cation gives a precipitate with potash alum?