Complete combustion of propane balanced equation?

• A. C3H8 + 3 O2 → 2 CO2 + 4 H2O
• B. C3H8 + 2 O2 → 3 CO2 + 4 H2O
• C. C3H8 + 4 O2 → 3 CO2 + 4 H2O
• D. C3H8 + 5 O2 → 3 CO2 + 4 H2O

Answer: (D)

In complete combustion of a hydrocarbon, the products are carbon dioxide and water. Balance the atoms step by step: propane has three carbons, so you form three CO2 molecules to use up all the carbon. That accounts for six oxygen atoms on the product side. Propane has eight hydrogens, so you form four H2O molecules to use up the hydrogens, adding four more oxygen atoms. So the total oxygen atoms needed on the product side are 6 plus 4, = 10. Each O2 molecule brings in two oxygen atoms, so you need 10/2 = five O2 molecules. Put it together: C3H8 + 5 O2 → 3 CO2 + 4 H2O. The other options don’t balance the oxygen atoms correctly, so they don’t represent a correct balanced equation for complete combustion.