Question

On passing a gas, ‘X’, through Nessler’s reagent, a brown precipitate is obtained. The gas ‘X’ is

• A. $\text{H}_2\text{S}$
• B. $\text{CO}_2$
• C. $\text{NH}_3$
• D. $\text{Cl}_2$

Answer 1

The Correct Option is C

To answer the question of which gas 'X' produces a brown precipitate with Nessler's reagent, let's examine the options and the reaction involved.

Nessler's Reagent Reaction: 

Nessler's reagent is a chemical solution used to detect the presence of ammonia (\(\text{NH}_3\)) in a sample. When ammonia gas is passed through Nessler's reagent, it reacts to form a brown precipitate of mercuric amido-iodide, which indicates the presence of ammonia.

The relevant reaction is:

\(\text{NH}_3 + 2\text{K}_2\text{HgI}_4 + 3\text{KOH} \rightarrow \text{HgO}\cdot\text{Hg(NH}_2\text{I}) + 7\text{KI} + 2\text{H}_2\text{O}\)

Option Analysis:

• \(\text{H}_2\text{S}\): When hydrogen sulfide is passed through Nessler's reagent, it does not form a brown precipitate. Instead, it reacts with metal ions to form black metal sulfides.
• \(\text{CO}_2\): Carbon dioxide does not react with Nessler's reagent to form any precipitate.
• \(\text{NH}_3\): This is the gas that reacts with Nessler's reagent to produce a brown precipitate, indicating the presence of ammonia.
• \(\text{Cl}_2\): Chlorine does not react with Nessler's reagent to produce a brown precipitate.

Therefore, the correct answer is the gas that reacts with Nessler's reagent to form a brown precipitate, which is \(\text{NH}_3\).

Conclusion:

Gas 'X' that forms a brown precipitate with Nessler's reagent is \(\text{NH}_3\), or ammonia.

Answer 2

Nessler’s Reagent Reaction:  
\(2K_2HgI_4 + NH_3 + 3KOH \rightarrow Hg_2(NH_3)_2I_2 + 7KI + 2H_2O\)

In the reaction above, the formation of a brown precipitate is characteristic of ammonia gas (\( NH_3 \)).

The Correct Answer is: \( NH_3 \)