Answer
Both oxygen and hydrogen exist as diatomic molecules in covalent bonds. However, each oxygen molecule combines with two hydrogen molecules to give a molecule of water. When Avogadro's law is applied ( the total number of atoms in the product should be the same as the total number in the reactants) the hydrogen+oxygen reaction equation does not balance. To balance the equation and to obey Avogadro's Law, two molecules of hydrogen must be combined with each molecule of oxygen to give two molecules of water.
Work Step by Step
Each molecule of oxygen and each molecule of hydrogen comprises two atoms respectively (bound covalently). However in the hydrogen + oxygen reaction that produces water each oxygen molecule bind with two hydrogen molecules. When one applies Avogadro's Law, two hydrogen molecules are required to balance the number of reactant atoms ( hydrogen and oxygen) with the number of atoms of those molecules in the product. The equation balances if two molecules of hydrogen are combined with one molecule of oxygen (proportionately) to give two molecules of water.
