Stoichiometry is the quantitative relationship between reactants and products in a chemical reaction. It rests on the mole concept and the balanced chemical equation, which obeys the law of conservation of mass — the number of atoms of each element must be identical on both sides.
Before any calculation, the formula of a compound is established from experiment. The percentage composition gives the mass percent of each element: $\%\,element = \dfrac{\text{mass of element in 1 mol}}{\text{molar mass}}\times100$. From this we derive two formulae:
- The empirical formula is the simplest whole-number ratio of atoms of each element. It is found by dividing each element's mass percent by its atomic mass to get mole ratios, then dividing by the smallest value.
- The molecular formula gives the actual number of atoms in one molecule. It is a whole-number multiple $n$ of the empirical formula, where $n = \dfrac{\text{molar mass}}{\text{empirical formula mass}}$.
For example, glucose has empirical formula $CH_2O$ (empirical mass $30$) and molar mass $180$, so $n = 180/30 = 6$ and the molecular formula is $C_6H_{12}O_6$.
A balanced equation such as $N_2 + 3H_2 \rightarrow 2NH_3$ reads in moles: $1\,mol$ $N_2$ reacts with $3\,mol$ $H_2$ to give $2\,mol$ $NH_3$. These coefficients are the stoichiometric ratios used to convert between any two species: convert the known mass to moles, apply the mole ratio, then convert back to mass or volume.
In practice reactants are rarely mixed in the exact stoichiometric ratio. The limiting reagent is the reactant that is completely consumed first; it determines the maximum amount of product. The other reactant, present in excess, is left over. To identify it, compute the moles of each reactant divided by its coefficient — the smallest value is the limiting reagent. All product calculations are based on the limiting reagent, never on the excess reactant.