Question

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

• A. \( \text{NaBr} \)
• B. \( \text{CaI}_2 \)
• C. \( \text{KNO}_3 \)
• D. \( \text{NH}_4\text{Cl} \)

Answer 1

The Correct Option is D

The reaction involved is:
\[2\text{NH}_4\text{Cl} + \text{MnO}_2 + 2\text{H}_2\text{SO}_4 \xrightarrow{\Delta} \text{MnSO}_4 + (\text{NH}_4)_2\text{SO}_4 + 2\text{H}_2\text{O} + \text{Cl}_2 \uparrow\]
In this reaction, chlorine gas (\( \text{Cl}_2 \)) is liberated, which has a greenish-yellow color.

Answer 2

Step 1: Understand the chemical situation
When manganese dioxide (\(\text{MnO}_2\)) reacts with concentrated sulfuric acid (\(\text{H}_2\text{SO}_4\)), it acts as an oxidizing agent and helps liberate certain gases from suitable salts. The question states that a greenish yellow gas is evolved when \(\text{MnO}_2\) and \(\text{H}_2\text{SO}_4\) are added to an unknown salt (A). We are asked to identify that salt.

Step 2: Recall relevant oxidation reactions of \(\text{MnO}_2\)
\(\text{MnO}_2\) in acidic medium can oxidize chloride ions (\(\text{Cl}^-\)) to chlorine gas (\(\text{Cl}_2\)). The reaction is:
\[ \text{MnO}_2 + 4\text{HCl} \rightarrow \text{MnCl}_2 + 2\text{H}_2\text{O} + \text{Cl}_2. \] Here, chlorine gas (\(\text{Cl}_2\)) is greenish yellow in color. This matches the color of the gas mentioned in the question.

Step 3: Source of hydrochloric acid (HCl)
If \(\text{MnO}_2\) and \(\text{H}_2\text{SO}_4\) are added to a chloride salt, the following reaction can occur: \[ 2\text{NaCl} + \text{H}_2\text{SO}_4 \rightarrow 2\text{HCl} + \text{Na}_2\text{SO}_4. \] The HCl formed then reacts with \(\text{MnO}_2\) to give chlorine gas. Hence, the salt (A) must be a chloride salt that can release HCl upon treatment with sulfuric acid.

Step 4: Identify which chloride salt gives the observed result
Among possible chloride salts — NaCl, KCl, NH₄Cl — all can form HCl when treated with concentrated sulfuric acid. However, the question specifically mentions a “greenish yellow gas” formed only in the presence of \(\text{MnO}_2\) and \(\text{H}_2\text{SO}_4\). That indicates oxidation of chloride ions to chlorine gas.
Thus, the chloride part of the salt is essential, but the question implies the identification of salt (A) by its full formula.

Step 5: Write the overall reaction with ammonium chloride
When \(\text{NH}_4\text{Cl}\) reacts with concentrated \(\text{H}_2\text{SO}_4\), it liberates \(\text{HCl}\) gas: \[ 2\text{NH}_4\text{Cl} + \text{H}_2\text{SO}_4 \rightarrow 2\text{HCl} + (\text{NH}_4)_2\text{SO}_4. \] The HCl produced then reacts with \(\text{MnO}_2\): \[ \text{MnO}_2 + 4\text{HCl} \rightarrow \text{MnCl}_2 + 2\text{H}_2\text{O} + \text{Cl}_2. \] The gas evolved (\(\text{Cl}_2\)) is greenish yellow, matching the question.

Step 6: Confirm reasoning
Therefore, the salt (A) must contain chloride ions and be capable of releasing HCl upon acid treatment. The presence of ammonia in the compound does not affect the oxidation, but ensures easy liberation of HCl. Thus, the salt (A) is ammonium chloride.

Final answer

\(\text{NH}_4\text{Cl}\)