Properties of Gas Produced by Treatment of Galena with Nitric Acid

Galena ($\text{PbS}$) reacts with dilute nitric acid ($\text{HNO}_3$) to yield lead(II) nitrate, elemental sulfur, water, and nitric oxide gas ($\text{NO}$):

$3\text{PbS(s)} + 8\text{HNO}_3\text{(aq)} \rightarrow 3\text{Pb(NO}_3\text{)}_2\text{(aq)} + 3\text{S(s)} + 2\text{NO(g)} + 4\text{H}_2\text{O(l)}$

The gas produced in this reaction is nitric oxide ($\text{NO}$).

Analyzing the properties of $\text{NO}$ gas:

1. Paramagnetism (Option A): Nitric oxide ($\text{NO}$) has a total of 15 electrons ($7 \text{ from N} + 8 \text{ from O}$), which is an odd number. According to Molecular Orbital Theory, the electronic configuration is: $(\sigma_{1s})^2 (\sigma^*_{1s})^2 (\sigma_{2s})^2 (\sigma^*_{2s})^2 (\sigma_{2p_z})^2 (\pi_{2p_x})^2 (\pi_{2p_y})^2 (\pi^*_{2p_x})^1$ The presence of one unpaired electron in the antibonding $\pi^*$ orbital makes $\text{NO}$ paramagnetic in the gaseous state. Thus, (A) is correct.

2. Geometry (Option B): $\text{NO}$ is a simple diatomic molecule. All diatomic molecules possess a linear geometry, not a bent geometry. Thus, (B) is incorrect.

3. Acidic/Neutral Nature (Option C): Nitric oxide ($\text{NO}$) does not react with water, acids, or bases to form salts; it is a neutral oxide (similar to $\text{N}_2\text{O}$ and $\text{CO}$). Thus, (C) is incorrect.

4. Color (Option D): Nitric oxide ($\text{NO}$) is a colorless gas in its gaseous state (it only turns brown upon reacting with oxygen in the air to form $\text{NO}_2$). Thus, (D) is correct.

Correct Options: (A) and (D)