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combustion analysis is based on the idea that all the carbon atoms that are now in the CO2 originally came from the sample, and all the hydrogen atoms in the water originally came from the sample. IF you use the mass percent of C in CO2 and the mass percent of H in H2O, you'll get the masses of those two elements that originally came from the vitamin C. (the remaining missing mass is from oxygen)
Turn those masses into moles, then divide by the smallest to find a whole number molar ratio. That's the empirical formula
How do I know which mass is from the oxygen in the vitamin C and which is from the oxygen present in all combustion reactions? And once I find the mass percents, do I multiply that by the mass of CO2 and water?
some of the oxygen in the CO2 comes from the sample, and some comes from the additional oxygen that all combustion reactions require. You don't know which, and it really doesn't matter. You know the compound contains C, H, and O. Using the mass percents of C and H allow you to assume the rest of the mass of the sample must have been the oxygen in the vitamin C itself