Which balanced equation correctly represents the combustion of methane in oxygen to form carbon dioxide and water?

• A. CH4 + 2 O2 -> CO2 + 2 H2O
• B. CH4 + O2 -> CO2 + H2O
• C. CH4 + 3 O2 -> CO2 + 3 H2O
• D. CH4 + 2 O2 -> CO2 + 2 H2O

Answer: (D)

Balancing by conserving atoms is the key. For methane burning in oxygen, aim to have the same number of each type of atom on both sides.

Carbon must be the same on both sides, so there is one CO2 produced. Methane has four hydrogens, so the products need four hydrogens, which comes from two H2O molecules, giving 2 H2O. Now count oxygen on the right: CO2 has two oxygens and 2 H2O has two more, totaling four oxygens. Those four oxygens must come from the left, which means two O2 molecules (each O2 has two oxygens). Putting it together yields CH4 + 2 O2 -> CO2 + 2 H2O, which balances all atoms. Using only one or three O2 would not balance the oxygen atoms.