Question

Which acid is used for brown ring test?

• A. Hydrochloric acid
• B. Carbonic acid
• C. Sulphuric acid
• D. Sulphurous acid

Answer 1

The correct option is C

Sulphuric acid

Explanation for correct option:

C: Sulphuric acid

1. In Brown ring test, saturated solution of Iron (II) sulphate along with an aqueous solution of Nitric acid is taken in a test tube.
2. To the mixture, Sulphuric acid is added in a drop wise manner.
3. After cooling the test tube, it is observed that a layer is formed of Nitroso ferrous sulphate between the two junction.
4. The chemical reaction followed as:
   $\underset{(\mathrm{Ferrous}\mathrm{sulphate})}{6{\mathrm{FeSO}}_4\left(\mathrm{aq}\right)}+\underset{(\mathrm{Sulphuric}\mathrm{acid})}{3{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)}+\underset{(\mathrm{Nitric}\mathrm{acid})}{2{\mathrm{HNO}}_3\left(\mathrm{aq}\right)}\to \underset{\left(\mathrm{Iron}\right(\mathrm{III})\mathrm{sulphate})}{3\mathrm{Fe}2{\left({\mathrm{SO}}_4\right)}_3\left(\mathrm{s}\right)}+4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+\underset{(\mathrm{Nitric}\mathrm{oxide})}{2\mathrm{NO}\left(\mathrm{g}\right)}$
5. In the above reaction, Ferrous sulphate reacts with Sulphuric acid and Nitric acid to form Iron (III) sulphate, Nitric oxide and water.
6. Further the reaction will complete as:
   $\underset{(\mathrm{Ferrous}\mathrm{sulphate})}{{\mathrm{FeSO}}_4\left(\mathrm{aq}\right)}+\underset{(\mathrm{Nitric}\mathrm{oxide})}{\mathrm{NO}\left(\mathrm{g}\right)}\to \underset{(\mathrm{Nitroso}\mathrm{ferrous}\mathrm{sulphate})}{{\mathrm{FeSO}}_4.\mathrm{NO}\left(\mathrm{aq}\right)}$
7. The remaining Ferrous sulphate reacts with Nitric oxide to form Nitroso ferrous sulphate.
8. Hence option C is correct. Sulphuric acid is used for brown ring test.