In Carius tube, an organic compound ' X ' is treated with sodium peroxide to form a mineral acid ' Y '. The solution of…

Step 1: Trace the Reaction Sequence

• Compound X is treated with sodium peroxide ($\ce{Na2O2}$, an oxidising agent) in a Carius tube
• A mineral acid Y is formed
• $\ce{BaCl2}$ is added to Y → precipitate Z
• Z is used for quantitative estimation of an extra element

Step 2: Identify the Precipitate Z

When $\ce{BaCl2}$ is added to a mineral acid:

• $\ce{BaCl2 + H2SO4 -> BaSO4(v) + 2HCl}$ → white precipitate (BaSO₄)

So Z = $\ce{BaSO4}$ (barium sulphate), which is used for the quantitative estimation of sulphur.

This means X contains phosphorus that gets oxidised to a phosphate acid? No — BaCl₂ + H₃PO₄ gives $\ce{Ba3(PO4)2}$, but that is less common in Carius method.

Actually Z = $\ce{BaSO4}$ → mineral acid Y = $\ce{H2SO4}$ → element estimated = Sulphur.

So X is a compound that contains sulphur (and gets oxidised to $\ce{H2SO4}$ by $\ce{Na2O2}$).

But wait — it says "an extra element" — meaning an element in addition to C, H, N, O. Let me reconsider: $\ce{Na2O2}$ oxidises phosphorus → $\ce{H3PO4}$ (mineral acid), and $\ce{BaCl2 + H3PO4 -> Ba3(PO4)2}$ (precipitate). $\ce{Ba3(PO4)2}$ is used to quantify phosphorus.

So: X contains phosphorus → oxidised to $\ce{H3PO4}$ (acid Y) → $\ce{BaCl2}$ added → $\ce{Ba3(PO4)2}$ (Z) → quantify phosphorus.

Step 3: Identify Compound X

X must contain phosphorus as an "extra element" (beyond C, H, N, O):

• (a) Cytosine: Contains C, H, N, O only — No P → ❌
• (b) Chloroxylenol: Contains C, H, O, Cl — No P → ❌
• (c) A nucleotide ✅ — Nucleotides contain C, H, N, O + phosphate group (P) → has the extra element phosphorus
• (d) Methionine: Contains C, H, N, O, S (sulphur, not phosphorus) → ❌

Step 4: Conclusion

Answer: (c) A nucleotide

Key Points to Remember:

• Nucleotides = nitrogenous base + sugar + phosphate group
• Phosphorus in nucleotides → oxidised by $\ce{Na2O2}$ → $\ce{H3PO4}$ → precipitated as $\ce{Ba3(PO4)2}$ with $\ce{BaCl2}$
• Carius method estimates halogens (AgX) and sulphur (BaSO₄) and phosphorus (Ba₃(PO₄)₂)
• Methionine contains S (not P); cytosine contains no heteroatom besides N and O