Calculating percentage composition from combustion data

I hope this subject isn't too obvious to include. Combustion analysis is a way to determine the percentage composition of elements in an organic molecule. The process is time-consuming and hence rarely done in research labs, unless it is a specialist lab which the sample has been sent to. Non-specialist labs typically just put a sample straight into modern spectrometers.

The logic used here is that moles of the product CO₂ equals the moles of C in the sample, while moles of product H₂O equals half the moles of H in the sample.

The moles of oxygen in the sample is what is left - assuming there are no other elements to take into account. In the above example, you can work out the moles of C and H in the sample, convert to weight, and subtract their weights from 9.83 mg. Convert the remaining weight to moles to get the moles of oxygen. 

With the moles of all three elements, you can convert into percentage composition.

The original process involved trapping the produced water in a hygroscopic agent, and the carbon dioxide in a strong base. Since carbon dioxide produces carbonic acid in water, the formation of an insoluble carbonate salt will, via Le Chatelier's principle, encourage the solvation of CO₂.

Structure of carbonic acid

Modern methods will separate the products via gas chromatography. Gas chromatography is a modern and complex way of separating and analysing compounds which can be vaporised without decomposition. I look forward to learning how they work.