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)

Conservation of mass means the same number of each type of atom must appear on both sides of a chemical equation. For this reaction, count hydrogen and oxygen on the left and on the right.

Left side: H2 provides 2 hydrogen atoms, O2 provides 2 oxygen atoms.

Right side (with water as product): each H2O has 2 hydrogens and 1 oxygen. With 1 H2O, you’d have 2 H and 1 O.

To balance oxygen, you need 2 water molecules on the right, which adds 2 oxygens and 4 hydrogens. Now you have 4 hydrogens on the right, so you must multiply the hydrogen reactant by 2 to supply 4 hydrogens. The balanced equation becomes 2 H2 + O2 -> 2 H2O.

This uses the smallest whole-number coefficients and keeps the product as water. The other options fail because they either leave oxygen unbalanced, alter the product to a different compound, or mismatch hydrogen atoms.