The stoichiometric reaction of the combustion of octane is C7H16 + 11O2 = 7 CO2 + 8H2O. Hence, for each mole of octane is equivalent to 11 moles of oxygen. 10.0 grams of octane divided by the molar mass (100g/mol) is equal to 0.1 mol. We multiply by 11 and the molar mass of oxygen (32g/mol), the answer is 35.2 grams oxygen.