Homework Help: Combustion reaction problem

1. Nov 30, 2013

alingy1

Hello everyone!

I have a question:

Let's say we have a 6g sample of a compound that has C, H and O. This sample is burned with excess oxygen.

I want to know if it is possible to recover the mass of O in the original compound from the mass of H2O and CO2 that will be produced (from combustion).

I also want to know if the mass of O in the original compound can be determined.

Here is my reasoning:

If we get the mass of C from CO2, that directly gives the mass of C in the compound. The same goes for hydrogen. We just subtract those two masses from 6g and get the mass of O in the compound.

So, I say that the mass of O in the original compound can be recovered and the mass of O in the original compound can be determined. What do you think?

2. Nov 30, 2013

Ygggdrasil

Yes, your approach to figuring out the mass of oxygen in the compound is correct. Also, you can calculate the mass of oxygen gas that reacted during the combustion simply by looking at mass balance. The mass of H2O and CO2 produced must be equal to the mass of your compound plus the mass of O2 burned.

3. Nov 30, 2013

Staff: Mentor

Not only it is correct, it is a basis of determining experimental formula for all oxygen containing compounds.

4. Nov 30, 2013

epenguin

Mass of O2 burned? :shy: Some mistake?

The problem says burned in excess oxygen, an unknown amount of which goes into the final H2O and CO2 as well as any O from the compound, so the data is insufficient by itself to tell you the O in the compound surely? We had a long exercise on this not long ago.

5. Nov 30, 2013

Staff: Mentor

You are given initial mass of the compound, no problem to find mass of oxygen in the compound, even if it reacted with the excess.

6. Nov 30, 2013

epenguin

OK I had missed we were given totalnorginalnweight from which the carbon and hydrogen oxides originated. I had been carrying from an earlier combustion analysis thread where thus was not given and it was quite an issue.

Relevant is
From "Chemistry 11" 2003 from Addison-Wesley quoted there
Footnote : Since elemental analysis data is derived from combustion analysis, it involves the combustion of the sample in a stream of oxygen. This means one can not measure the amount of oxygen present in the sample and hence it can only be assumed based on the missing %. Note that routine or standard elemental analysis is for the elements C, H and N. As a cross check, before commiting to assuming that the missing % is oxygen, check to see if any other data suggests any other element is present (e.g. Br or Cl). If no other data is provided (or available), then all you can do is assume that it is oxygen. If N was found it the elemental analysis, it would be reported as a % N.

7. Dec 1, 2013

Staff: Mentor

Yes, I remember the other thread, but it was a different situation.

8. Dec 1, 2013

Yanick

Epenguin, I remember that thread, it was a doozy. The difference in that thread was that the OP was not given the mass of the sample that was burned, so determining O content was impossible. I was trying to get the OP to at least get something out of the discussion in the way of knowledge because I couldn't see how to solve for a "nice" answer.

Also, to clarify for any who have questions, you do not determine O content in the sample by calculating the O content of the water and carbon dioxide products because we are combusting with excess oxygen which results in the O content of the products being the sum of the O content in the unknown compound and the oxygen gas in the atmosphere of the combustion system. Typically you determine O content in the original sample as outlined here; by mass balance which requires knowledge of the mass off the combusted unknown sample.

9. Dec 1, 2013

epenguin

Yes, and the fact that was not given reinforces my impression then that it was an artificially invented academic exercise that was even based on a mistake! :rofl:

10. Dec 1, 2013

Yanick

I agree. I just didn't want to be the one that said it :)