How much product will a reaction produce? How much reactant is needed? Stoichiometry answers these questions. It is the calculation of the quantities of reactants and products in a chemical reaction. The word comes from the Greek stoicheion meaning element and metron meaning measure. It is based on the balanced chemical equation and the mole concept. It is the quantitative backbone of chemistry.

The balanced equation gives the mole ratio. In the reaction 2H2 + O2 -> 2H2O, two moles of hydrogen react with one mole of oxygen to produce two moles of water. The coefficients are the key. They tell you the ratio in which substances react and are produced. If you know the amount of one substance, you can calculate the amounts of all the others.

The steps are straightforward. Write the balanced equation. Convert the given quantity to moles. Use the mole ratio from the equation to find the moles of the desired substance. Convert back to the desired unit. The conversion may involve molar mass for solids and liquids or the ideal gas law for gases. The limiting reactant is the one that runs out first, and it determines the theoretical yield.

Stoichiometry is used in industry to calculate raw material requirements, predict yields, and minimize waste. It is used in the laboratory to prepare solutions and run reactions. It is used in environmental science to calculate emissions. It is a fundamental skill for any chemist.