Identification of Gas X and Brown Ring Complex Y

The brown ring test is a standard qualitative test used for the detection of nitrate ($\text{NO}_3^-$) or nitrite ($\text{NO}_2^-$) ions in aqueous solution.

1. Formation of Gas $X$: In the brown ring test, nitrate ions react with iron(II) sulfate ($\text{FeSO}_4$) in acidic conditions, where $\text{Fe}^{2+}$ reduces $\text{NO}_3^-$ to nitric oxide ($\text{NO}$) gas: $\text{NO}_3^- + 3\text{Fe}^{2+} + 4\text{H}^+ \longrightarrow \text{NO}\text{(g)} + 3\text{Fe}^{3+} + 2\text{H}_2\text{O}$ Therefore, the gas $X$ is $\text{NO}$ (nitric oxide).

2. Formation of Brown Ring Complex $Y$: The liberated nitric oxide gas ($\text{NO}$) binds with the unreacted hydrated ferrous ions present in the freshly prepared ferrous sulphate solution to form a dark brown coordination complex: $[\text{Fe}(\text{H}_2\text{O})_6]^{2+} + \text{NO} \longrightarrow [\text{Fe}(\text{H}_2\text{O})_5(\text{NO})]^{2+} + \text{H}_2\text{O}$ In simplified formulaic representation, the brown ring complex $Y$ is written as $[\text{Fe}(\text{NO})]\text{SO}_4$.

Therefore, the correct identification for $X$ and $Y$ is $\text{NO}$ and $[\text{Fe}(\text{NO})]\text{SO}_4$, respectively.

Correct Option: A