$1.367$ of an organic compound containing carbon, hydrogen and oxygen was combusted in a stream of air to yield $3.002$g of $CO_2$ and $1.640$g of $H_2O$. The molecular mass of the compound was found to be $120$ a.m.u. Find its empirical formula and molecular formula.

I calculated the empirical and molecular formulae of the compound and they were found to be $C_3H_8O$ and $C_6H_{16}O_2$.

I want to know if all of the Carbon in $CO_2$ is that of $C_6H_{16}O_2$? If this is so then why is it $CO_2$? It should be $C_6O_2$, and I've the same confusion for Hydrogen.


All carbon in $\ce{CO2}$ is from $\ce{C6H16O2}$, right.

It is $\ce{CO2}$ because organic compounds with only $\ce{C}$, $\ce{H}$ and $\ce{O}$ give $\ce{CO2}$ and $\ce{H2O}$ upon complete combustion in $\ce{O2}$.

Did you perhaps mean the oxygen atom of the $\ce{O2}$? Depending on the molecule, the oxygen atoms could be in either $\ce{O2}$, or $\ce{H2O}$ or both.

If you have, for example $\ce{R-C-O-H}$, the oxygen atom will stay with the hydrogen atom and go in an $\ce{H2O}$ molecule. Otherwise, if it's in $\ce{R-C=O}$ or $\ce{R-C-O-C-R}$, it will go in a $\ce{CO2}$ molecule.

Disclaimer: The basis for the above paragraph is the bond energies of $\ce{C-O}$, $\ce{C=O}$ and $\ce{O-H}$. I don't have evidence from experiments carried out to specifically identify those. Such an experiment would probably make use of an isotope of oxygen-16 and mass spectrometry.

By the way, that molecular formula is really strange. I've never seen something like that and a search brought no results (besides having more than one molecule being considered as a single).

Equation for reaction as requested:

$\ce{C6H16O2 + 9O2 -> 6CO2 + 8H2O}$

  • $\begingroup$ what could be the chemical equation of the above reaction? $\endgroup$ – Samama Fahim May 20 '13 at 18:07
  • $\begingroup$ Yes, of course. $\endgroup$ – Samama Fahim May 20 '13 at 19:44
  • $\begingroup$ Oh really sorry! I think I didn't see that.. $\endgroup$ – Samama Fahim May 20 '13 at 20:39
  • $\begingroup$ @SamamaFahim It's okay :) Anything else? $\endgroup$ – Jerry May 20 '13 at 20:39
  • $\begingroup$ Thank you Jerry :). Well, is $C_6$ a molecule of carbon in $C_6H_{16}O_2$? And what's the molecular form of carbon? I think $C_2$. $\endgroup$ – Samama Fahim May 20 '13 at 20:49

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