[mac227]

I am having trouble figuring this out.  We did an experiment where we lit a burner and heated a tube filled with Ca and NaCl (a 3:1 ratio) for about 5 minutes.  We repeated this 3 times in succession and we weighed the fuel burner and tube before and after each trial.  The tube was supposed to collect the H2O and Co2 produced.  Now we have to answer the following question:   


"Using the masses obtained, determine the emperical formula of the fuel.  The changes in mass of the dessicant tube should equal the mass of water produced.  The change in mass of the ascarite tube should equal the mass of CO2 produced.  The change in mass of the burner should equal the mass of fuel burned."

Our changes in tube mass are: 
trial 1) .448
trial 2) .692
trial 3).312

Changes in fuel burner mass:
trial 1) -1.87
trial 2)-2.249
trial 3)-2.571

My question is how am I supposed to derive the empirical formula from this information??
From the question it seems as though there were supposed to weigh 2 tubes, but in the experiment we only weighed 1 single tube (which collected the CO2 and H2O).  Should I use the change in burner mass to account for the CO2 produced or something??

[DevaDevil]

I assume the fuel is a pure hydrocarbon, without any other elements, burning in pure oxygen.

So you have the general (unbalanced) equation of:

a C_xH_y + b O₂ --> (a*x) CO₂ + (a*y/2) H₂O

the mass of your hydrocarbon you know from your burner mass, the mass of water + carbon dioxide you know from the tube. However, you do not know the ratio of the 2, which complicates things:


I have a solution, but I do have 1 small problem... your measurements.
see, if you assume your tube contains both water AND CO₂ generated by the reaction, it has to have MORE mass increase than the fuel lost; this because the oxygen from the air reacts with it and increases the total mass...
in your shown experiments it seems like the tube gains less mass than the fuel loses. which means you lose a product somewhere (you cannot just lose mass)

[mac227]

Yes, I am sure that not all the CO2 was collected.  We had a smaller CO2 absorbant tube at the end of our dessicant tube which was supposed to absorb some CO2 (Im sure this isnt an effective method of trapping CO2,  I doubt it even captures a significant %)  My problem is that we weighed the single tube and CO2 absorbant tube as one tube (which we were supposed to do)  But wouldn't i need to weigh them seperately in order to figure out my calculations?

I also am having trouble following your equation "a CxHy + b O2 --> (a*x) CO2 + (a*y/2) H2O"

And I am not sure our burner fuel was a pure hydrocarbon or alcohol or some other type of fuel.  I know that it had an internal wick and we needed to keep a cap on the opening because our instructor told us it would evaporate if we didnt which makes me think its some type of alcohol?

[DevaDevil]

simple hydrocarbons can also evaporate quickly (even quicker than alcohols)

the problem is, if the fuel is not a pure hydrocarbon, then there is no solution, as you did not keep track of how much oxygen was used.

the formula I gave was of the complete combustion of a hydrocarbon with molecular formula C_xH_y. Each C in the hydrocarbon ends up in the carbon dioxide. Therefore, the number of carbon dioxide molecules formed is a (the number of fuel molecules) * x (the number of carbon atoms per molecule), this goes identically for water.

[mac227]

So if the fuel was alcohol (CH3CH2OH)  there would be no solution because it is not a hydrocarbon?  What is the most common hydrocarbon or if you had to guess what hydrocarbon do you think our instructor would of had us use?

[DevaDevil]

if it is a mono-alcohol (aka one O per molecule) it will have a solution, but if it is a compound with an unknown quantity of C, H and O atoms, you cannot know, because of the parameter of oxygen you did not measure.

and was the fuel liquid, or a gas?