When $\mathrm{MnO}_2$ and $\mathrm{H}_2 \mathrm{SO}_4$ is added to a salt $(\mathrm{A})$, the greenish yellow gas liberated as salt (A) is :

<p>When a salt reacts with $\mathrm{MnO}_2$ and concentrated $\mathrm{H}_2 \mathrm{SO}_4$, the type of gas evolved depends on the anion present in the salt. In this case, a greenish yellow gas is evolved, which indicates the formation of chlorine gas ($\mathrm{Cl}_2$). $\mathrm{Cl}_2$ gas is greenish-yellow in color and is typically produced from halide salts (namely chlorides, bromides, or iodides) when they are oxidized. Among the given options, we need to identify a salt that contains a halide which can produce $\mathrm{Cl}_2$ upon reaction with $\mathrm{MnO}_2$ and concentrated $\mathrm{H}_2 \mathrm{SO}_4$.</p> <p>Option A ($\mathrm{NH}_4 \mathrm{Cl}$) contains chloride ions, and when heated with $\mathrm{MnO}_2$ and concentrated $\mathrm{H}_2 \mathrm{SO}_4$, it can undergo a redox reaction where the chloride ions are oxidized to $\mathrm{Cl}_2$ gas. The corresponding reaction can be represented as follows:</p> <p>$$\mathrm{MnO}_2 + 4\mathrm{HCl} \longrightarrow \mathrm{MnCl}_2 + \mathrm{Cl}_2 + 2\mathrm{H}_2\mathrm{O}$$</p> <p>Option B ($\mathrm{CaI}_2$) contains iodide ions, which would lead to the liberation of iodine or $\mathrm{I}_2$, not a greenish yellow gas.</p> <p>Option C ($\mathrm{KNO}_3$) contains nitrate ions and does not produce a halogen gas upon reaction with $\mathrm{MnO}_2$ and $\mathrm{H}_2 \mathrm{SO}_4$.</p> <p>Option D ($\mathrm{NaBr}$) contains bromide ions which would lead to the formation of bromine ($\mathrm{Br}_2$), a reddish-brown gas, not greenish yellow.</p> <p>Therefore, the correct answer is <strong>Option A</strong> ($\mathrm{NH}_4 \mathrm{Cl}$), since it is the salt that can produce a greenish yellow gas ($\mathrm{Cl}_2$) when reacted with $\mathrm{MnO}_2$ and concentrated $\mathrm{H}_2 \mathrm{SO}_4$.</p>