Conceptual--Combustion of a sample 1. The problem statement, all variables and given/known data When a sample is burned in a combustion train, the percent oxygen in the sample cannot be determined directly from the mass of water and carbon dioxide. Why? 2. Relevant equations 3. The attempt at a solution I'm working through my chemistry book solo and I'm almost done with chapter 2! However, I got to this question and it was something I wondered about when I was previously working "determination of simplest and molcular formula" problems. Because I would first find the mass of C in CO2 and then the mass of H in H2O, then subtract it from the mass of the sample that was combusted, and I didn't understand why I couldn't also compute the masses of oyxgen and just add them together, instead of going the other route, but the book worked it the other way so I didn't bother. If anyone can lead me to the answer or outright tell me that would be nice!