Question

In the brown ring complex, the oxidation state of iron is:

Answer 1

Nitrates give brown fumes with concentrated $H_{2}S{O}_4$due to formation of $N{O}_2$. This is intensified upon adding copper turnings. Nitrate ion is confirmed by adding an aqueous solution of the salt to $FeS{O}_4$and pouring concentrated $H_{2}S{O}_4$slowly along the sides of the test tube, which produces a brown ring around the walls of the tube, at the junction of the two liquids caused by the formation of ${\left[Fe{\left(H_{2}O\right)}_5\right(NO\left)\right]}^{2+}$.