Treatment of a gas ' X ' with a freshly prepared ferrous sulphate...

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

In this test, nitric oxide (NO) gas is evolved which reacts with the freshly prepared ferrous sulphate ($\text{FeSO}_4$) solution to form a brown ring of nitroso ferrous sulphate.

The gas $X$ is NO.

The reaction is given by:

$\text{FeSO}_4 + \text{NO} \rightarrow [\text{Fe(NO)}]\text{SO}_4$

The actual composition of the brown ring complex is $[\text{Fe(H}_2\text{O})_5(\text{NO})]\text{SO}_4$, which is often represented as $[\text{Fe(NO)}]\text{SO}_4$.

Thus, $X$ is NO and $Y$ is $[\text{Fe(NO)}]\text{SO}_4$.

Answer: NO and $[\text{Fe(NO)}]\text{SO}_4$