Question

The sodium extract of an organic compound on acidification with acetic acid and addition of lead acetate solution gives a black precipitate. The organic compound contains:
A.Nitrogen
B.Halogen
C.Sulphur
D.Phosphorus

Answer 1

Hint:The organic compounds contain elements like nitrogen, sulphur, halogen, phosphorus. These elements can be detected by Lassaigne’s test. In lassaigne’s test, sodium fusion extract is prepared and is tested for the detection of elements.

Complete step by step answer:
-Heteroatoms such as nitrogen, phosphorus, sulphur are present in an organic compound. In organic qualitative analysis, we need to perform tests to confirm the presence of such atoms.
-Lassaigne’s test is used to detect the presence of such elements. In this test, the elements present in covalent form are converted into ionic form by fusing the compound with sodium metal. The reaction of sodium metal with the elements takes place as follows-
$Na + C + N \to NaCN$
$2Na + S \to N{a_2}S$
$Na + X \to NaX$
Where $X$ is a halogen.
-These elements if present in the given sample form such a complex with sodium metal. Cyanide, sulphide and halide of sodium so formed are extracted from the fused mass by boiling it in distilled water. This extract is known as sodium fusion extract.
-This extract can be further treated with different reagents to confirm the presence of above elements.
-When sodium fusion extract is acidified with acetic acid and lead acetate, a black precipitate if obtained indicates the presence of sulphur in the given sample. The reaction can be stated as follows-
${S^{2 - }} + P{b^{2 + }} \to PbS$
-The black precipitate is due to the formation of lead sulphide.
The correct option is C.

Note:
Lassaigne’s test is a test used for qualitative analysis. Along with these elements, carbon and hydrogen are detected by heating the compound with copper oxide. Quantitative analysis of these elements can also be done to estimate the percentage composition of elements in a compound.