Why is nitrogen chemically inert?

Which of the following statements is true regarding nitrogen?

$FeSO_4$ forms a brown ring with which of the following?

Nitric acid forms an oxide of nitrogen on reaction with $P_{4}O_{10}$. Write the reaction involved. Also,write the resonating structures of the oxide of nitrogen formed.

$P_2O_3 + H_2O \rightarrow X$
$Red \ P_4 + \text{alkali} \rightarrow Y$
$X$ and $Y$ are oxoacids of phosphorus. The number of $P-OH$ bonds in $X$ and $Y$ respectively is:

$HNO_3 + P_4O_{10} \to 4HPO_3 + X$. $X$ is