# Why weren't the moles of oxygen calculated the same way as C?

Combustion analysis of a $\rm1.500~g$ sample of ascorbic acid yields $4.023\rm~g$ of $\ce{CO2}$ and $\rm0.96~g$ of $\ce{H2O}$. What is the empirical formula of ascorbic acid?

Actually my question is that in many solutions I saw it's solved like that:

In $\ce{CO2}$ the moles of C is calculated as $$\frac{4.023}{44.02}=0.9139\rm~mass=1.0976~g$$

and the $\ce{H2O}$ is solved with getting out the H moles with same way that C was solved and the mass is $$\rm m~H=0.10761~g$$ the mass of O will be $$\rm 1.500-(0.10761~g-1.0976~g)=0.29489~g~moles=0.1842$$ My question is why didnt we use the same procedure we did with H and C to compute the moles with O? Why did we make it in the last step?

• Actually guys, let me edit this. Since it's a homework question with some effort and solution. – M.A.R. Feb 16 '16 at 18:02
• @IͶΔ Well, it was in dire need of editing ;) – Mithoron Feb 16 '16 at 18:16

You have done combustion analysis, so you have introduced a large amount of oxygen to the ascorbic acid sample. Because of this, you can't know how much of the oxygen in the $\ce{CO2}$ and $\ce{H2O}$ you produced actually came from the acid sample and how much came from the atmosphere.