Problem
An organic compound X is subjected to systematic elemental analysis via Lassaigne's test. The following observations are made:
- Lassaigne's extract + FeSO4 + NaOH, boiled, cooled + H2SO4 → no blue precipitate
- Lassaigne's extract + sodium nitroprusside → purple/violet color
- Lassaigne's extract (after boiling with dilute HNO3) + AgNO3 → white precipitate which dissolves in dilute NH3
Identify the elements present in compound X and explain each step.
Step-by-Step Solution
Step 1 — Interpret test 1 (no blue precipitate): The Prussian blue test for nitrogen gave no result. This means either: (a) Nitrogen is absent, OR (b) Nitrogen IS present but along with sulfur (so NaSCN formed instead of NaCN; no NaCN → no Prussian blue). We cannot conclude yet — need test 2 to clarify.
Step 2 — Interpret test 2 (purple color with nitroprusside): Sodium nitroprusside gives purple color with Na2S (sulfide). This confirms: Sulfur (S) is present. Now revisiting test 1: S is present. If N were also present, NaSCN would form and test 1 would fail (no Prussian blue). The fact that test 2 confirms Na2S (not NaSCN) means no separate NaCN and no NaSCN — i.e., Nitrogen is absent (otherwise NaSCN would form and the nitroprusside test might give a weaker result for S). Conclusion: S is present, N is absent.
Step 3 — Interpret test 3 (white precipitate with AgNO3, soluble in dilute NH3): White AgX precipitate soluble in dilute NH3 = AgCl → Chlorine (Cl) is present. The extract was first boiled with HNO3 to remove Na2S (which would give black Ag2S interference). After that, AgNO3 gave white AgCl.
Final Conclusion: Compound X contains: Sulfur (S) and Chlorine (Cl). Nitrogen is absent.