A stoichiometric coefficient is a number used in a chemical equation to indicate the relative amounts of reactants and products involved in a reaction. It shows how many molecules or moles of a substance participate in the reaction. For example, in the equation 2H2 + O2 → 2H2O, the coefficients 2 and 1 indicate that two molecules of hydrogen react with one molecule of oxygen to produce two molecules of water. These coefficients are essential for balancing chemical equations, ensuring that the law of conservation of mass is upheld. By adjusting the stoichiometric coefficients, chemists can accurately represent the proportions of substances involved, which is crucial for calculations in stoichiometry, such as determining yields and reactant requirements.