Question 25 of 34intermediate💡 UnderstandShort Answer2 marks

Why is it necessary to use acetic acid and not sulphuric acid for acidification of sodium extract for testing sulphur by lead acetate test?

Correct Answer

During the test for sulphur, the sodium fusion extract is acidified with acetic acid and lead acetate is added to it. The appearance of a black precipitate of lead sulphide indicates the presence of sulphur, following the reaction: S2−+Pb2+→PbSS^{2-} + Pb^{2+} \rightarrow PbS (Black).

It is necessary to use acetic acid instead of sulphuric acid because sulphuric acid would react with lead acetate to form a white precipitate of lead sulphate. This white precipitate would interfere with the observation of the black lead sulphide precipitate, leading to a false result. Acetic acid provides the necessary acidic medium without reacting with the lead acetate reagent.

Exercise: EXERCISES | Q: 8.31
For More Understanding

Explanation

The textbook context states that the sodium fusion extract is acidified with acetic acid before adding lead acetate to test for sulphur, which forms a black precipitate of PbS. The reason sulphuric acid is avoided (though implied by standard chemical principles rather than explicitly detailed in the provided text) is that it would introduce sulphate ions that react with lead acetate to form a white precipitate of lead sulphate, interfering with the actual test for sulphur.

Solution Steps

  1. Step 1: The sodium fusion extract is acidified with acetic acid.

  2. Step 2: Lead acetate is added, reacting with S2−S^{2-} ions to form a black precipitate of PbSPbS.

  3. Step 3: Sulphuric acid is avoided because it reacts with lead acetate to form a white precipitate of lead sulphate, which interferes with the test.

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