Combustion and Solving for Empirical and Molecular Formula

[K Honeychurch 1K]

#1 of Quiz 1 Preparation Fall 2012 says that a compound containing carbon, hydrogen, and oxygen undergoes complete combustion. It looks like the answer assumes that all of the oxygen in the products (CO2 and H2O) came from the compound, but combustion always has O2 as a reactant so some of the oxygen in the products must have come from the O2, right? Thanks in advance.

[Patrick Chin 1F]

Actually you are right, all combustion does require oxygen. This also applies to the problem, and you can find the amount of oxygen used in the combustion by subtracting the mass of the sample from the total mass of the products. .3664g + .1500g = .5164g of product was formed, but there was only .2500g of sample. Therefore, by conservation of mass, there was .5164g-.2500g = .2664g of O2 in the reaction. Dividing this by the molar mass of O2 you get .2664 / (15.999 x 2) = .008326 mol O2. You need to subtract this O2 from the total O2 you calculate from the products to get the answer for this question.