Why does $PCl_3$ fume in moisture?

Define the order of $d_{P-O}$ (single bond length) in the given compounds $X$ and $Y$:
$P_4 \xrightarrow{O_2} 'X'$
$P_4 \xrightarrow{O_2 \text{ (excess)}} 'Y'$

Which acid is formed by $P_2O_3$?

Which element does not show allotropy:

In which oxidation state,group $15$ elements act as Lewis base?

Sodium nitrite with hydrochloric acid gives water along with two nitrogen oxides. They are