Butane Combustion Analysis Demo for a High School Chemistry Class

[mishima]

Hi, I'm a high school chemistry teacher trying to develop a lab experience demonstrating combustion analysis. This is usually something we go through theoretically during discussion of percent composition and empirical formula. I'd like to actually perform one.

I was wondering if this could be done with a simple butane torch, or even a simple bic lighter?

I would setup the glassware to collect the vapors of the flame as follow:

--Open flame leading to long flute
--Flute going into 2-hole rubber bung in 150 mL erlynmeyer flask partially filled with calcium chloride (water absorber)
--Intake goes all the way to the bottom, submerged in the absorber
--Outlet runs to 2-hole rubber bung in second 150 mL erlynmeyer partially filled with sodium hydroxide (CO2 absorber)
--Again intake goes all the way to the bottom, submerged
--Outlet goes to vacuum

I've only ever seen this in textbook pictures and so am not really aware of any practical considerations here, especially the quantities of the absorbers required.

 

[Bystander]

--Open flame leading to long flute

"Flute," or flue? You've not begun or ended the combustion, so you're looking for qualitative demonstration of formation of CO₂ and H₂O?

 

[mishima]

Not sure of the correct term, but its basically an inverted glass funnel.

I am certainly not looking for a qualitative demonstration of the formation of those products, we have already studied combustion itself on the qualitative level. The point is to see how combustion analysis can lead to an empirical formula for the hydrocarbon being burned. Masses of CO₂ and H₂O are converted into mol C and mol H. A pseudoformula with fractional subscripts is then turned into an empirical formula with whole integer subscripts. Despite butane having a formula easily stated using organic nomenclature rules, I think there is value in demonstrating the procedure.

 

[Bystander]

You're looking for C_nH_{(2n + 2)} in a poorly defined mixture,

simple butane torch

, which is properly LP, a mixture of C_(1-5) saturated and unsaturated hydrocarbons. i.e., you are not going to get "whole integer subscripts." You might want to look at/google "fuel gas combustion calorimetry."

 

[mishima]

No reason it has to be butane, just looking for something on hand. How about just burning a candle?

 

[Bystander]

a candle?

Same thing, a mixture.