How would you balance C8 H18 + O2 = CO2 +H20?
You have octane. Under complete combustion there MUST be 8 equivalents carbon dioxide as a consequence. Water is the other product; how many equiv water result? And how many equiv oxygen gas are used?
If I were to combust heptane, I would formulate the combustion rxn this way:
You have octane. How are you going to modify the equation?