Question

$F{e}^{2+}\left(aq\right)+N{O}_3{}^{-}\left(aq\right)+H_{2}S{O}_4\left(conc\right)⟶$ Brown ring.
The oxidation number of iron in brown ring complex is:

Answer 1

The formula of the brown ring is $\left[Fe\right(H_{2}O{)}_{5}NO]S{O}_4$. Here $NO$ is in $+1$ state that's why the oxidation number of iron is $+1$