# Why won't my propane combust when I spark it?

I need to run an experiment and have some pretty basic questions. I'm trying to get combustion of propane in a controlled environment. I understand that that looks like this:

$$\ce{C3H8 + 5O2 -> 3CO2 + 4H2O}$$

Meaning I need 5 times more $\ce{O2}$ molecules than $\ce{C3H8}$. But, I want to do this based on the volumes of the gases at STP. Do I just go with 5 times the volume of $\ce{O2}$ to the volume of $\ce{C3H8}$? Do the gas laws require that I take into account the size of a molecule?

And seeing that $\ce{O2}$ comprises about $\pu{21\%}$ of the volume of air, I would need to multiply the volume of oxygen by roughly 5 to get the correct volume of air needed...yes?

That gets me a 25 to 1 ratio between air and propane by volume. So, I tried it, but I couldn't get it to burn.

Doing further research, I see something called the Limiting Oxygen Concentration, but I think either I don't understand it, or this isn't the problem.

I also found that the lower and upper limits of flammability for propane are $\pu{2.15\%}$ and $\pu{9.60\%}$ respectively. My concentration is roughly $\pu{4\%}$, so this shouldn't be the issue either.

The only other thing I can think of is that my spark isn't "good enough". Is there a minimum amount of heat required from my spark to ignite the mixture?

Anyone have any ideas as to what I am missing here? Any help would be much appreciated!

• Hello and welcome to Chemistry.SE. If you have any questions about the site please feel free to take the short tour or visit the help center. Best of luck with your question! – airhuff Apr 20 '17 at 4:37
• Do you mean that you actually mix some volumes of air and propane, and then try to ignite it? Then it is probably a good thing that some of your components was faulty, otherwise you could blow your head off and get arrested for that. – Ivan Neretin Apr 20 '17 at 8:05
• @IvanNeretin Necroing this, but your order of actions is amusing. Would they arrest the severed head or the headless body? – Tristan Jun 27 '18 at 18:26
• @Tristan Depends on the jurisdiction, I guess. – Ivan Neretin Jun 27 '18 at 18:29