In a test tube containing a salt, a few drops of dilute H₂SO₄ was added, which gave colourless vapours having the smell of vinegar. The vapours turned the blue litmus paper red. Identify the correct anion from the following:

When dilute sulfuric acid (H₂SO₄) is added to a solid salt, the anion present in the salt can react with the acid to form a volatile acid.

Observation 1:
Colour‑less vapours are produced that have the characteristic smell of vinegar.

The smell of vinegar is the smell of acetic acid (CH₃COOH). Therefore the volatile acid formed must be acetic acid, which indicates that the anion in the original salt was the acetate ion (CH₃COO⁻).

Observation 2:
The vapours turn blue litmus paper red.

This confirms that the vapour is acidic, consistent with acetic acid. Other possible anions give different products:

• Carbonate (CO₃²⁻) would produce carbon dioxide (CO₂), a colourless gas without a vinegar smell; CO₂ forms a weakly acidic solution but does not give a distinct vinegar odour.
• Sulphate (SO₄²⁻) would not produce a volatile acidic compound; the mixture would remain largely unchanged and no characteristic odour would be observed.
• Sulphide (S²⁻) would generate hydrogen sulphide (H₂S), which has a rotten‑egg smell, not a vinegar smell, and the gas would turn litmus black rather than red.

Since the only anion that yields a colourless, vinegar‑smelling, acidic vapour is acetate, the correct identification of the anion is:

Acetate, CH₃COO⁻