A light bluish-green crystalline compound responds to the following tests:
(i). Its aqueous solution gives a brown precipitate on reaction with alkaline `K_2[HgI_4]` solution.
(ii). Its aqueous solution gives a blue colour with `K_3[Fe(CN)_6]` solution.
(iii). Its solution in hydrochloric acid gives a white precipitate with `BaCl_2` solution. Identify the ions present and suggest the formula of the compound.

Explanation

Its aqueous solution gives a brown precipitate or colouration with alkaline `K_2[HgI_4]` solution. This suggests ammonium ions.
Its aqueous solutions gives a blue colour with `K_2[Fe(CN)_6]` solution. This suggests ferrous ions. Its solution in hydrochloric acid gives a white precipitate with `BaCl_2` solution. This suggest sulphate ions. The formula of the compound can be `FeSO_4(NH_4)_2SO_4.6H_2O` (Mohr's salt.)