QJEEM24Che19f: JEE Main 2024 Chemistry - Class 11 JEEM24Che19f Solution: When and is added to a salt , the greenish yellow gas liberated …

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 $\mathrm{H}_2 \mathrm{SO}_4$ .

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:

$\mathrm{MnO}_2 + 4\mathrm{HCl} \longrightarrow \mathrm{MnCl}_2 + \mathrm{Cl}_2 + 2\mathrm{H}_2\mathrm{O}$

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.

Option C ( $\mathrm{KNO}_3$ ) contains nitrate ions and does not produce a halogen gas upon reaction with $\mathrm{MnO}_2$ and $\mathrm{H}_2 \mathrm{SO}_4$ .

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.

Therefore, the correct answer is Option A ( $\mathrm{NH}_4 \mathrm{Cl}$ ), since it is the salt that can produce a greenish yellow gas ( $\mathrm{Cl}_2$ ) when reacted with $\mathrm{MnO}_2$ and concentrated $\mathrm{H}_2 \mathrm{SO}_4$ .