In the balanced equation CH4 + 2 O2 → CO2 + 2 H2O, how many molecules of O2 react with each molecule of CH4?

In the balanced equation CH4 + 2 O2 → CO2 + 2 H2O, how many molecules of O2 react with each molecule of CH4?

• A. 1
• B. 2
• C. 3
• D. 4

Answer: (B)

The equation shows how many molecules participate, not just which substances are present. For each molecule of methane, two molecules of oxygen react. You can see this by balancing atoms: methane has 4 hydrogens and 1 carbon, and the products contain CO2 (which has 2 oxygen atoms) plus 2 H2O (which adds 2 more oxygen atoms), totaling 4 oxygen atoms. Each O2 molecule provides 2 oxygen atoms, so you need 2 O2 molecules to supply those 4 oxygens. Using only one O2 would leave oxygen under-supplied and the equation unbalanced.

The equation shows how many molecules participate, not just which substances are present. For each molecule of methane, two molecules of oxygen react. You can see this by balancing atoms: methane has 4 hydrogens and 1 carbon, and the products contain CO2 (which has 2 oxygen atoms) plus 2 H2O (which adds 2 more oxygen atoms), totaling 4 oxygen atoms. Each O2 molecule provides 2 oxygen atoms, so you need 2 O2 molecules to supply those 4 oxygens. Using only one O2 would leave oxygen under-supplied and the equation unbalanced.