Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers and students. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I have butene being burned in a fuel rich engine and I can't seem to balance this reaction

$$\ce{C4H8 + a~ (O2 + 3.76N_{2}) -> b ~CO2 + c~ H2O + d~ C4H8 + e~ (3.76N_{2})}$$

When I try write equations for the elements, I get 5 equations and 4 unknowns which can't be solved. Here are my equations:

Carbon: $4=b+4d$

Oxygen: $2a=2b+c$

Nitrogen: $3.76a=3.76e;\quad a=e$

Hydrogen: $2a=2c+8d$

Edit: I'm not sure if this makes a different but it says that water in the combustion products is removed before the dry analysis of the exhaust. The DRY analysis has a composition of 14.95% $CO_{2}$, 0.75% $C_{4}H_{8}$, 0% $CO$, 0% $H_{2}$, 0% $O_{2}$, and balance $N_{2}$ (I don't know what that means)

share|improve this question
re last sentence: it means you've got a fifth equation for your five unknowns. By the way, at the end of your reaction equation, should the 3(...) actually be e(...)? – EnergyNumbers Sep 24 '12 at 11:05
as EnergyNumbers pointed out DRY analysis gives you have a fifth equation: $b/d = 14.95/0.75 = 19.933$. Solving those will give you $a = e = 5.994$, $b=3.331$, $c=5.326$ and $d=0.167$. – mythealias Oct 28 '12 at 18:52
See this video for a complete description of a matrix-based approach. It starts by showing how to set up the matrix. It then takes each step of reducing the matrix and extracting coefficients. – Rich Apodaca Feb 24 '13 at 1:01
@RichApodaca your link describe a generic algorithm to solve the problem, which is fine, but probably not very helpful to the specific question as here. – F'x Jul 22 '13 at 10:43

Balanced equation for the combustion of $\ce{C4H8}$ is $$\ce{C4H8 + 6O2 -> 4CO2 + 4H2O}$$ Nitrogen is included only to indicate that it is present in the medium with some specific ratio to the oxygen amount --well technically it is included to make question look complicated.

$\ce{C4H8}$ in the products side is included to indicate that not all of it can be combusted but some escapes. And it really is arbitrary, depends on the the engineering of combustion room etc -which is why you have 4 equations, 5 unknowns: d is free to vary. But you are given the dry analysis to fix its value.

Looking at the equations given, you have

b = c = 4(1-d)
a = e = b + c/2 = 6(1-d) = 3b/2

And dry analysis says ratio of $\ce{CO2}$:$\ce{C4H8}$ is 14.95:0.75 = 19.93 (b/d) after the combustion. And this is the last equation you need.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.