An inorganic compound ' \(X\) ' on treatment with concentrated \(H _2 SO _4\) produces brown fumes and gives dark brown …

An inorganic compound ' \(X\) ' on treatment with concentrated \(H _2 SO _4\) produces brown fumes and gives dark brown ring with \(FeSO _4\) in presence of concentrated \(H _2 SO _4\). Also compound ' \(X\) ' gives 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 deep blue coloured solution, Compound ' \(X\) ' is: