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Q.
$P _{4} O _{10}$ is not used to dry $NH _{3}$ gas because
The p-Block Elements - Part2
Solution:
$P _{4} O _{10}$ is acidic and $NH _{3}$ is basic, so they react with each other in presence of moisture giving ammonium phosphate. Hence, $P _{4} O _{10}$ can't be used for drying of $NH _{3}$.
$P _{4} O _{10}+12 NH _{3}+6 H _{2} O \rightarrow 4\left( NH _{4}\right)_{3} PO _{4}$