Reaction of Nitric Acid with Phosphorus Pentoxide

To determine the correct options, let us first identify the nitrogen-containing compound formed in the given reaction.

Step 1: Identification of the Compound

Phosphorus pentoxide ($\text{P}_4\text{O}_{10}$) acts as a strong dehydrating agent. When it reacts with concentrated nitric acid ($\text{HNO}_3$), it dehydrates nitric acid to form dinitrogen pentoxide ($\text{N}_2\text{O}_5$) and metaphosphoric acid ($\text{HPO}_3$):

$4\text{HNO}_3 + \text{P}_4\text{O}_{10} \longrightarrow 2\text{N}_2\text{O}_5 + 4\text{HPO}_3$

Thus, the nitrogen-containing compound formed is $\text{N}_2\text{O}_5$.

---

Step 2: Evaluation of Options

• Option (A): "can also be prepared by reaction of $\text{P}_4$ and $\text{HNO}_3$" The reaction of white phosphorus ($\text{P}_4$) with concentrated nitric acid produces phosphoric acid ($\text{H}_3\text{PO}_4$) and nitrogen dioxide ($\text{NO}_2$), not $\text{N}_2\text{O}_5$: $\text{P}_4 + 20\text{HNO}_3 \longrightarrow 4\text{H}_3\text{PO}_4 + 20\text{NO}_2 + 4\text{H}_2\text{O}$ Therefore, option (A) is incorrect.

• Option (B): "is diamagnetic" In $\text{N}_2\text{O}_5$, all electrons are paired. In the gaseous phase, its structure is $\text{O}_2\text{N}-\text{O}-\text{NO}_2$, and in the solid phase, it exists as an ionic crystal $[\text{NO}_2]^+[\text{NO}_3]^-$. Since there are no unpaired electrons in either form, $\text{N}_2\text{O}_5$ is diamagnetic. Therefore, option (B) is correct.

• Option (C): "contains one $\text{N-N}$ bond" The molecular structure of $\text{N}_2\text{O}_5$ consists of two $\text{NO}_2$ groups bridged by an oxygen atom ($\text{O}_2\text{N}-\text{O}-\text{NO}_2$). Thus, it contains an $\text{N-O-N}$ linkage and no direct $\text{N-N}$ bond. Therefore, option (C) is incorrect.

• Option (D): "reacts with $\text{Na}$ metal producing a brown gas" $\text{N}_2\text{O}_5$ is a strong oxidizing agent and reacts with alkali metals such as sodium ($\text{Na}$) to form sodium nitrate ($\text{NaNO}_3$) and nitrogen dioxide ($\text{NO}_2$), which is a characteristic brown gas: $\text{N}_2\text{O}_5 + \text{Na} \longrightarrow \text{NaNO}_3 + \text{NO}_2 \uparrow$ Therefore, option (D) is correct.

---

Conclusion

The correct options are B and D.