A mole ratio is a way to compare the amounts of different substances involved in a chemical reaction. It is derived from the coefficients of a balanced chemical equation, which represent the number of moles of each reactant and product. For example, in the reaction 2H2 + O2 → 2H2O, the mole ratio of H2 to O2 is 2:1, meaning two moles of hydrogen react with one mole of oxygen. Mole ratios are essential for stoichiometry, which helps chemists calculate how much of each substance is needed or produced in a reaction. By using mole ratios, scientists can predict the outcomes of reactions and ensure that they have the correct proportions of reactants, minimizing waste and maximizing efficiency.