Why Can't Percent Oxygen Be Directly Determined from Combustion Products?

[carmen77]

Conceptual--Combustion of a sample

Homework Statement

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?

Homework Equations

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!

 

[Borek]

What are the most common elements present in organic substances?

 

[carmen77]

That would be C-C bonds or C-H bonds often in combination with oyxgen, nitrogen, sulfur, and other elements but most that I have come across so far have only included the elements C, H, and O.

 

[Borek]

most that I have come across so far have only included the elements C, H, and O.

I suppose here lies answer to your question. Here, and in the meaning of the word "directly".

If a compound contains C, H and O, can you calculate how much oxygen it contains knowing ONLY masses of H₂O and CO₂? No, you need mass of the sample. And probably that's what they mean.

 

[carmen77]

I don't know much about chemistry but is that because the sample undergoes a reaction in the presence of Oxygen O2 and a spark and produces CO2 and H20, so therefore, one couldn't obtain the mass of oyxgen in the sample from the addition of the mass of oyxgen in CO2 and the mass of oxygen in H20, because the sample also reacted with O2.

 

[Borek]

This is all in simple stoichiometry.

Imagine your burnt sample (of unknown mass) gave 44g of carbon dioxide and 18g of water.

Calculate how much carbon was in your sample.

Calculate how much hydrogen was in your sample.

Try to calculate how much oxygen was in your sample.

Show your calculations, it should be easier to analyse the problem.

 

[carmen77]

44g CO2 * (1 mole CO2/44g CO2)=1 mole CO2 * (2 mole O/1 mole CO2)*(16g O/1 mole O)=

32g O in 44g of CO2

Did the same for C and got 12 g C in 44g of CO2

18g H20 * (1 mole H20/18g H2O)= 1 mole H20 * (1 mole O/1 mole H2O) * (16g O/1 mole O)=

16g O in 18 g H2O

2g H in 18g H2O

Now if I add them all together, why wouldn't that equal the mass of the sample? 32g O+12g C+ 16g O+2g H= 62g

 

[Borek]

Why do you think all oxygen comes from the original sample?

Try to solve other question: you have 14 grams of ethylene (C₂H₄) reacting with excess oxygen. What will be the mass of carbon dioxide and water produced?

 

[carmen77]

Ah, so the oyxgen comes from the sample and the oyxen it reacts with and that's why you can't find the oxygen of the sample from a calculation of the oyxgen in the products.

 

[Borek]

You've got it