Combustion Analysis Problem

[Karina Vasquez 1D]

A compound consists only of Carbon, Hydrogen, and Oxygen. 8.272 g of CO2 and 4.515 g of H2O are produced during complete combustion of 3.765g of the compound. a) What is the empirical formula of the compound? b) What is the molecular formula if the molar mass of the compound is 300. 47 g/mol?
I know that solving this problem you have to convert given masses to moles to find the moles of CO2 and H2O. Then I would have to use the molar ratio to find the mass of Carbon, Hydrogen, and Oxygen. What I'm confused on is the mass 3.765. Is that the mass of Oxygen, would I not find the mass of Oxygen because it is already given to me?

[Chem_Mod]

From the given mass of CO2, you can find the moles of carbon. Similarly, from the given mass of H2O, you can find the moles of hydrogen. These cannot tell you the moles of oxygen because combustion requires O2 to be a reactant. You would be unable to tell if the oxygen in the carbon dioxide came from either the air or the compound itself.

To find the moles of oxygen, you first have to find the mass of O. Since you are given the compound's total mass, you must subtract the mass of carbon and hydrogen from the total compound mass (convert the moles above to mass). This tells you how much oxygen is in your compound, and from there you can continue with your mole ratios of C, H, and O.