Question

Black precipitate in the detection of sulphur with lead acetate and acetic acid is due to formation of:
(A) $ {\text{P}}{{\text{b}}_{\text{2}}}{\text{S}} $
(B) $ {\text{PbS}} $
(C) $ {\text{Pb}}{{\text{S}}_{\text{2}}} $
(D) $ {\text{PbS}}{{\text{O}}_{\text{4}}} $

Answer 1

Hint: Organic compounds contain a number of hetero atoms like nitrogen, sulphur, phosphorous and halogens. The presence of these atoms is detected by various tests, the most commonly used one of which is Lassaigne’s test. In this process, a sodium fusion extract of the given compound is prepared which is then tested for the presence of the hetero atoms. It forms a compound with $ P{b^{2 + }} $ and $ {S^{2 - }} $ .

Complete step by step solution:
An organic compound has almost all of its atoms covalently bonded. As a result, to test their detection during organic qualitative analysis, it is important to first convert the covalent form of the atom to an ionic form so that they can show their characteristic tests. Lassaigne’s test used for the detection of elements like nitrogen, sulphur, phosphorus and halogens also performs using the same course of action. The elements are converted into the ionic form by the fusion of the compound with sodium metal. The reactions of the elements with sodium metal occur as:
 $ Na + C + N \to NaCN $
 $ 2Na + S \to N{a_2}S $
 $ Na + X \to NaX $ (Halogens)
These extracts formed are then treated with various reagents for detecting the presence of these elements. For testing the presence of sulphur, the extract is acidified using acetic acid and then lead acetate is added. A black precipitate is formed if sulphur is present and the reaction occurring is given as
 $ {S^{2 - }} + P{b^{2 + }} \to PbS $
Hence, the correct option is B.

Note:
For the preparation of sodium extract, we heat a small piece of sodium metal till it forms a shining state in a fusion tube. The given compound is then added to this tube and strongly heated. The tube when red hot is taken into distilled water. The water is then filtered and the filtrate obtained is known as the sodium extract.