In Carius tube, an organic compound 'X' is treated with sodium peroxide to form a mineral acid 'Y'. The solution of BaCl$$_2$$ is added to 'Y' to form a precipitate 'Z'. 'Z' is used for the quantitative estimation of an extra element. 'X' could be

We need to identify organic compound X that, when treated with sodium peroxide in a Carius tube, forms a mineral acid Y, which on addition of $$BaCl_2$$ gives a precipitate Z used for quantitative estimation.

In the Carius method for elemental analysis, the organic compound is heated with a strong oxidizing agent (sodium peroxide, $$Na_2O_2$$) in a sealed tube. Different elements are converted to different products:

- Sulfur (S) is oxidized to sulfate ($$SO_4^{2-}$$)

- Halogens are converted to halide ions

- Phosphorus is converted to phosphate

The mineral acid Y is formed from the oxidation product. When $$BaCl_2$$ is added to Y and forms precipitate Z, the precipitate must be $$BaSO_4$$ (barium sulfate), because:

$$BaSO_4$$ is a white precipitate that is insoluble in water and dilute acids. This is the standard test for sulfate ions, and it is used for quantitative estimation of sulfur in organic compounds. The mineral acid Y is therefore $$H_2SO_4$$ (sulfuric acid).

Identify compound X (must contain sulfur).

Among the options:

(A) Cytosine - $$C_4H_5N_3O$$ - contains C, H, N, O but NO sulfur.

(B) A nucleotide - nucleotides contain C, H, N, O, P but typically NO sulfur.

(C) Methionine - $$C_5H_{11}NO_2S$$ - this is an amino acid that CONTAINS SULFUR.

(D) Chloroxylenol - $$C_8H_9ClO$$ - contains C, H, Cl, O but NO sulfur.

Only methionine contains sulfur. When methionine is treated with $$Na_2O_2$$ in a Carius tube, the sulfur is oxidized to $$Na_2SO_4$$, which on acidification gives $$H_2SO_4$$ (mineral acid Y). Adding $$BaCl_2$$ gives $$BaSO_4$$ precipitate (Z), which is weighed for quantitative estimation of sulfur.

The correct answer is Option 3: Methionine.