# Number of moles in a chemical equation with an amount of substance given

How many moles of carbon dioxide are there in $20.5~\mathrm{g}$ of oxygen gas given this reaction: $\ce{C15H30 + O2 -> CO2 + H2O}$

So I figured out the balanced equation is $\ce{2C15H30 + 45O2 -> 30CO2 + 30H2O}$ I thought that the ratio between moles of oxygen gas and that of carbon dioxide would be $45/30$. And then I know that the moles in $20.5~\mathrm{g}$ of oxygen gas is $0.640~\mathrm{mol}$. The answer I got from cross multiplying is different from the answer key, which is $19.2\ \mathrm{mol}$.

I dont see any problem with my method. So I am wondering if there is anything wrong with the answer key or I misunderstood the question.

• Hi and welcome to chemistry.stackexchange.com. Feel free to take a tour of the site. I improved the formatting of your post with MathJax; for more information on how to do so yourself, consult the help center or visit this or this meta-post. Your question is a homework question, therefore I am pointing you to our homework policy. By the way, what is the actual question? ‘How many moles of $\ce{CO2}$ are there in $\ce{O2}$ gas?’ – Jan Nov 15 '15 at 17:51

How many moles of carbon dioxide are there in $20.5~\mathrm{g}$ of oxygen gas given this reaction: $\ce{C15H30 + O2 -> CO2 + H2O}$
Assuming that there is enough hydrocarbon for complete consumption of the $\ce{O2}$, then oxygen is used in ratio of $2:1$ for the products $\ce{CO2}$ and $\ce{H2O}$. Since you started out with 20.5 g (20.5/32 = 0.641 mol) of $\ce{O2}$, then (2/3)*0.641 = 0.427 mol $\ce{CO2}$.
I notice that if you start out with 20.9 grams of $\ce{O2}$, then you get 19.2 g $\ce{CO2}$. So something is wrong with the whole problem as you have given it.