Which equation represents the balanced combustion of methane?

• A. CH4 + O2 -> CO2 + 2 H2O
• B. CH4 + 2 O2 -> CO2 + 2 H2O
• C. CH4 + 3 O2 -> CO2 + 4 H2O
• D. 2 CH4 + O2 -> CO2 + 2 H2O

Answer: (B)

Balancing atoms in a combustion reaction is the idea. Methane has 1 carbon and 4 hydrogens, so the products must include 1 CO2 and 2 H2O to keep the C and H counts equal on both sides. The oxygen atoms also need to balance: CO2 has 2 O and each H2O has 1 O, so with 2 H2O there are 2 more O, giving 4 O in total on the products. Those come from two O2 molecules, which supply 4 oxygen atoms overall. So the balanced equation is CH4 + 2 O2 → CO2 + 2 H2O. Using fewer or more O2 would upset the balance of O (and thus the whole equation), and changing the amount of methane would also disrupt C and H balance.