JEE Advanced 2021 Paper 2 Q16 Chemistry Inorganic Chemistry Qualitative Analysis Medium

JEE Advanced 2021 Paper 2 · Q16 · Qualitative Analysis

Among the following, the brown ring is due to the formation of

  1. A. [Fe(NO)$_2$(SO$_4$)$_2$]$^{2-}$
  2. B. [Fe(NO)$_2$(H$_2$O)$_4$]$^{3+}$
  3. C. [Fe(NO)$_4$(SO$_4$)$_2$]
  4. D. [Fe(NO)(H$_2$O)$_5$]$^{2+}$
Reveal answer + step-by-step solution

Correct answer:D

Solution

Brown-ring test for NO$_3^-$: NaNO$_3$ + FeSO$_4$ + conc. H$_2$SO$_4$ liberates NO, which displaces one water from the [Fe(H$_2$O)$_6$]$^{2+}$ aquo-complex to form the brown complex [Fe(H$_2$O)$_5$(NO)]$^{2+}$ (often written [Fe(NO)(H$_2$O)$_5$]$^{2+}$).

Net (one Fe centre, +2 overall, with NO behaving as NO$^+$ and Fe formally as Fe(I)): [Fe(H$_2$O)$_6$]$^{2+}$ + NO $\rightarrow$ [Fe(NO)(H$_2$O)$_5$]$^{2+}$ + H$_2$O. The brown ring at the interface contains exactly one NO and five aqua ligands, matching option (D).

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