For the complete combustion of methane, how many O2 molecules are required per mole of CH4?

• A. 1 O2
• B. 2 O2
• C. 3 O2
• D. 4 O2

Answer: (B)

Complete combustion needs enough oxygen to convert all carbon to carbon dioxide and all hydrogen to water. Start by balancing CH4 with oxygen to form CO2 and H2O. The carbon from methane becomes CO2, so one methane gives one CO2. There are four hydrogens, so you need two water molecules to hold those hydrogens, giving 2 H2O. Count oxygens on the product side: CO2 has 2 O, and 2 H2O has 2 more O, totaling 4 O atoms. Oxygen must come from O2 molecules, and each O2 provides 2 O atoms, so you need two O2 molecules to supply 4 O atoms. The balanced equation is CH4 + 2 O2 -> CO2 + 2 H2O, so two O2 molecules are required per methane.