In Carius tube, an organic compound '$\mathrm{X}$' is treated with sodium peroxide to form a mineral acid 'Y'.

The solution of $\mathrm{BaCl}_{2}$ is added to '$\mathrm{Y}$' to form a precipitate 'Z'. 'Z' is used for the quantitative estimation of an extra element. '$\mathrm{X}$' could be