Stoichiometry
Definition
The mathematics of chemical reactions: using a balanced equation to predict how much reactant you need or how much product you will make.
Technical definition
Stoichiometry is the quantitative study of the relative amounts of reactants and products in chemical reactions, based on the mole ratios given by balanced chemical equations and the conservation of mass.
Example
From 2H2 + O2 -> 2H2O, stoichiometry shows that 4 grams of H2 (2 mol) need 32 grams of O2 (1 mol) to make 36 grams of water (2 mol).
Common confusion
Stoichiometry assumes the reaction goes exactly as written. In real labs, incomplete reactions and side reactions mean actual yields are usually lower than the stoichiometric prediction.