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?

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

But you do know that the mass of your ascorbic acid is the mass of all the carbon plus the mass of all the hydrogen, which you calculated, plus the mass of oxygen. The difference in mass between what you have calculated and what you started with is the mass of oxygen that was in the ascorbic acid.

If there were any other elements in your sample, such as sulphur or nitrogen, those amounts would also have to be calculated before finally working out how much mass you haven't accounted for which must have been oxygen.

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  • $\begingroup$ I really appreciate your helping and very thankful , but in fact I used the fact that like 0.25 mol of AgCl means 0.25 mol of Ag and 0.25 mol of Cl and it also doesn't work. are my calculations wrong you think or my principle is typically wrong? $\endgroup$ – 119 j7fly Feb 16 '16 at 18:51
  • $\begingroup$ Where has the AgCl come from? Seems like a completely different question. $\endgroup$ – Spontification Feb 16 '16 at 19:39
  • $\begingroup$ Im so stupid sorry because of my confusion I just said without indicting that its a different question .yah its different,actually its the question posted here chemistry.stackexchange.com/questions/13751/… – I thought if I used the mass og AgCl to have its moles and the say Cl has 2.103/143.3=0.0147 mol ^is this right?doing the same way that C moles have in previous question?.Im really sorry to mix the thing up but I have a crucial exam tomorrow. $\endgroup$ – 119 j7fly Feb 16 '16 at 20:18
  • $\begingroup$ I think you are correct. because the ratio of Ag:Cl is 1:1, 0.0147 moles of AgCl means 0.0147 moles of Ag+ and the same of Cl-, just as in this question 0.91 moles of CO2 means 0.91 moles of carbon atoms. $\endgroup$ – Spontification Feb 16 '16 at 20:30
  • $\begingroup$ You reference to the AgCl question, is answered using a different method. The final result mass of Cl should be the same if you use the method in this question, but the intermediate steps will be completely different so you can't compare your progress. $\endgroup$ – Spontification Feb 16 '16 at 20:41

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