Combustion analysis is a way of determining empirical/molecular formulas of unknown substances. It is based on the following chemical reaction:
Unknown substance (i.e.) CxHyOz + O2(g) =>CO2 (g) + H20(g)
If you remember, the key to finding out the empirical formula is to know the actual mass of each of atoms in the unknown sample. To do this we use two tricks: 1) Conservation of Mass-the mass of each atom into the chemical reaction (from reactants) must equal the mass of that atom out (products) and 2) Law of Difference- if you know the mass of two things, you can find out third if you know total.
Here is how to do it.
Example:
The characteristic odor of pineapple is due to ethyl butyrate. Combustion of 2.78 mg of ethyl butyrate yields 6.32 mg of CO2 and 2.58 mg of H20. Determine the empirical formula of ethyl butyrate.