Balance the equation: H2 + O2 -> H2O.

• A. H2 + O2 -> H2O
• B. 2 H2 + O2 -> 2 H2O
• C. H2 + 2 O2 -> H2O2
• D. 4 H2 + O2 -> 2 H2O

Answer: (B)

Balancing means making sure the same number of each type of atom is on both sides of the equation, honoring the conservation of mass. Start with the unbalanced form and count the atoms: left has 2 hydrogen atoms per H2 and 2 oxygen atoms per O2, while the right has H2O with 2 hydrogen and 1 oxygen. To fix oxygen, put a 2 in front of the water on the right, giving 2 H2O. Now the right side has 4 hydrogen and 2 oxygen atoms, so hydrogen is unbalanced. To fix that, put a 2 in front of the hydrogen gas on the left. This yields 2 H2 + O2 -> 2 H2O. Now both sides have 4 hydrogen and 2 oxygen atoms, so the equation is balanced with the smallest whole-number coefficients.

The balanced equation is: 2 H2 + O2 -> 2 H2O.