Identify the gas evolved with sodium hydroxide

In salt analysis, when an ammonium salt is treated with sodium hydroxide, ammonia gas is evolved:

$\mathrm{NH_4^+ + OH^- \rightarrow NH_3 \uparrow + H_2O}$

So, gas $X$ is most likely $\mathrm{NH_3}$.

Test for ammonia gas

A standard confirmatory test for ammonia is Nessler's reagent, which is an alkaline solution of potassium tetraiodomercurate(II):

$\mathrm{K_2HgI_4 + KOH}$

Ammonia gives a brown coloured precipitate with Nessler's reagent.

Evaluate the options

• A: $X=\mathrm{NH_3}$ and $Y=\mathrm{HgO}$
  Incorrect. $\mathrm{HgO}$ is not the standard reagent giving the characteristic brown precipitate in this test.

• B: $X=\mathrm{NH_4Cl}$ and $Y=\mathrm{KOH}$
  Incorrect. $\mathrm{NH_4Cl}$ is not a gas here; ammonia is evolved.

• C: $X=\mathrm{NH_3}$ and $Y=\mathrm{K_2HgI_4 + KOH}$
  Correct. This is Nessler's reagent, which gives a brown precipitate with ammonia.

• D: $X=\mathrm{HCl}$ and $Y=\mathrm{NH_4Cl}$
  Incorrect. HCl is not evolved when ammonium salt is treated with NaOH.

Conclusion

Therefore,

$X=\mathrm{NH_3}, \quad Y=\mathrm{K_2HgI_4 + KOH}$

So the correct option is C.