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Q.
The reaction of $P_4$ with $X$ leads selectively to $P_4O_6$. The $X$, is
IIT JEEIIT JEE 2009The p-Block Elements - Part2
Solution:
In limited supply of oxygen, phosphorus is oxidised to its lower oxide $P_4O_6$ while excess of oxygen gives $P_4O_{10}$. A mixture of $O_2\,$ and $\, N_2$ is used for controlled oxidation of phosphorus into $P_4O_6$