Concept:In elemental organic analysis (such as combustion analysis), elements like C, H, N, and Halogens are trapped or chemically reacted to form measurable distinct products. Oxygen behaves differently.
Formula:$$ \%\text{O} = 100 - (\%\text{C} + \%\text{H} + \%\text{N} + \text{...}) $$
Solution:- During combustion analysis, an organic compound is burned in an excess supply of external Oxygen gas.
- Because vast amounts of external oxygen are flooded into the system to force combustion, there is absolutely no chemical way to distinguish which oxygen atoms in the resulting \( \text{CO}_2 \) and \( \text{H}_2\text{O} \) came from the original organic compound, and which came from the external gas supply.
- Therefore, Oxygen cannot be detected directly. It is always calculated mathematically by the "method of difference" (subtracting all other element percentages from 100%).
Why other options are incorrect:- Chlorine, Nitrogen, Phosphorous: All of these can be directly detected and quantified via Lassaigne's test or other direct precipitation/titration methods.
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