Chemistry 2010 JAMB Past Questions - Question 31: To a solution of an unknown compound, a little dilute tetraoxosulphate (VI) acid was added with some freshly prepared iron (II) tetraoxosulphate (VI) solution. The brown ring observed after the addition of a stream of concentrated tetraoxosulphate (VI) acid confirmed the presence of

Correct Answer

D

Explanation

Based on the information provided, the presence of a **brown ring** after adding concentrated sulfuric acid confirms the presence of **nitrate ions (NO3-)** in the unknown compound.

Here's why:

- The initial addition of dilute sulfuric acid and iron(II) sulfate solution serves as a test for nitrates.
- If nitrate ions are present, they react with iron(II) ions to form a pale yellow complex, [Fe(H2O)5NO]2+.
- When concentrated sulfuric acid is added slowly, it forms a denser layer below the aqueous solution.
- If the pale yellow complex is present, it reacts with the concentrated sulfuric acid and nitrite ions formed from the reduction of nitrate by iron(II) ions, generating the characteristic **brown ring** at the junction of the two layers.

This test, known as the **brown ring test**, is a specific and sensitive method for detecting nitrate ions. Several other ions can interfere with the test, but the specific conditions you described (using dilute and concentrated sulfuric acid and freshly prepared iron(II) sulfate) suggest a high likelihood of the presence of nitrates.