JEE Advanced 2016 Paper 2 Chemistry: Paragraph 2

A pale green crystalline double salt $\mathbf{X}$ is analysed. Its aqueous solution gives a white precipitate with $\mathrm{BaCl_2}$ that is insoluble in dilute HCl, a dark blue precipitate with $\mathrm{K_3[Fe(CN)_6]}$, and the smell of ammonia on warming with NaOH solution. Analysis shows that one formula unit of $\mathbf{X}$ contains two $\mathrm{NH_4^+}$, two $\mathrm{SO_4^{2-}}$, one iron ion and six molecules of water of crystallisation, so that $\mathbf{X}$ is stable in air and is used as a primary standard in redox titrations. Take $M(\mathbf{X})=392.1\ \mathrm{g\,mol^{-1}}$. A solution of $\mathbf{X}$ is mixed with $\mathrm{NaNO_3}$, and conc. $\mathrm{H_2SO_4}$ is carefully added down the side of the tube, which gives a brown ring at the junction due to a complex $[\mathrm{Fe(H_2O)_5NO}]^{2+}$; in this complex the nitrosyl ligand is $\mathrm{NO^+}$, the iron is high-spin and the five water molecules are neutral.