Selective Oxidation of White Phosphorus to Phosphorus Trioxide

To selectively synthesize phosphorus trioxide ($\text{P}_4\text{O}_6$), white phosphorus ($\text{P}_4$) must be oxidized under controlled conditions with a limited supply of oxygen.

1. Combustion in Excess Oxygen:
   When white phosphorus reacts with pure oxygen ($\text{O}_2$, dry or moist) in excess, it undergoes complete and vigorous oxidation to form phosphorus pentoxide ($\text{P}_4\text{O}_{10}$):
   $\text{P}_4 + 5\text{O}_2 \longrightarrow \text{P}_4\text{O}_{10}$

2. Selective Oxidation to $\text{P}_4\text{O}_6$:
   To stop the oxidation at the $+3$ oxidation state and prevent the formation of $\text{P}_4\text{O}_{10}$, the concentration of oxygen must be diluted using an inert gas such as nitrogen ($\text{N}_2$). A mixture containing a limited amount of $\text{O}_2$ diluted with $\text{N}_2$ (such as air or a controlled $\text{O}_2/\text{N}_2$ gas stream) allows the reaction to proceed selectively to phosphorus trioxide:
   $\text{P}_4 + 3\text{O}_2 \xrightarrow{\text{diluted with }\text{N}_2} \text{P}_4\text{O}_6$

Therefore, $\mathbf{X}$ is a mixture of $\text{O}_2$ and $\text{N}_2$.

Correct Option: B