In CH₄ + 2O₂ → CO₂ + 2H₂O, why is it 2 O₂ and not 1? I tried balancing it with just one and the atoms didn’t add up.
Count the oxygen atoms the products need. One CO₂ holds two oxygen atoms and each of the two H₂O molecules holds one, so the products need four oxygen atoms in total. Oxygen comes in pairs as O₂, so it takes two O₂ molecules: CH₄ + 2O₂ → CO₂ + 2H₂O, with every atom accounted for on both sides.
Accepted answer
Count the oxygen atoms needed on the product side: one CO₂ needs 2 oxygen atoms, and two H₂O molecules need another 2 oxygen atoms - that’s 4 oxygen atoms in total. Since oxygen gas comes as O₂ (pairs), you need exactly 2 O₂ molecules to supply all 4. It’s conservation of mass: every atom on the left has to show up on the right.