Which of the products indicates the presence of sulphur atom in an organic compound, in qualitative…
Which of the products indicates the presence of sulphur atom in an organic compound, in qualitative elemental analysis?
\(\mathrm{Fe}_2 \mathrm{SO}_4\)
ZnS
\(\mathrm{MgS}\)
\(\mathrm{PbS}\)
Solution
In qualitative elemental analysis of organic compounds (Lassaigne test) sulphur atom of the organic molecule gets converted into water soluble salt \(\mathrm{Na}_2 \mathrm{~S}\). It is tested with lead acetate \(\left(\mathrm{CH}_3 \mathrm{COO}\right)_2 \mathrm{~Pb}\) solution to give a black ppt. of \(\mathrm{PbS}\).
\(\mathrm{Na}_2 \mathrm{~S}(a q) \xrightarrow{\left(\mathrm{CH}_3 \mathrm{COO}\right) \mathrm{Pb}} \underset{\text { Black }}{\mathrm{PbS} \downarrow}+2 \mathrm{CH}_3 \mathrm{COONa}\)