Which one of the following combines with $Fe(II)$ ions to form a brown complex?

In $P_4O_6$,how many oxygen atoms are attached to each phosphorus atom?

Nitrogen $(I)$ oxide $(N_2O)$ is produced by:

The gases liberated when zinc reacts with dilute and conc. $HNO_3$ respectively are

Which reaction$(s)$ among the following produce dinitrogen?
$1. NH_4Cl(aq) + NaNO_2(aq) \longrightarrow$
$2. NH_2CONH_2 + 2H_2O \longrightarrow$
$3. Ba(N_3)_2 \longrightarrow$
$4. NH_4Cl + Ca(OH)_2 \longrightarrow$

Concentrated $HNO_3$ turns brown on standing due to the formation of: