Question

# The reaction of ${P_4}$ with X leads selectively to ${P_4}{O_6}$. The X is :a.) dry ${O_2}$b.) a mixture of ${O_2}$ and ${N_2}$c.) moist ${O_2}$d.) ${O_2}$ in presence of aq. NaOH

Hint: This reaction requires less supply of oxygen. So, some other element will be used that will competitively inhibit the full supply of oxygen.

For this, first, we will see the reaction of formation of ${P_4}{O_6}$.
${P_4} + 3{O_2}\xrightarrow[{{N_2}}]{{{O_2}}}{P_4}{O_6}$
Phosphorus trioxide is prepared by heating phosphorus in a limited supply of oxygen and nitrogen.
Oxygen supply is provided in a limited quantity because if it is in more quantity then ${P_4}{O_6}$ will further oxidize to ${P_4}{O_6}$.

Thus, option b.) is the correct answer.