The stoichiometry of a reaction is the ratio in which the substances react and form, shown by the numbers in front of the formulae in the balanced equation. You can deduce it from the masses of the reactants and products.
Method: first change each mass into moles by dividing by its relative formula mass Mr (or Ar for an element). Then divide all the moles by the smallest number of moles. If needed, multiply so that every number is a whole number. This simplest whole-number ratio gives the numbers in the balanced equation. It is the same arithmetic used to find an empirical formula.
Example: 2.4 g of magnesium (Ar 24) reacts with 1.6 g of oxygen (O2, Mr 32) to make 4.0 g of magnesium oxide (Mr 40). The moles are 0.1 mol of magnesium, 0.05 mol of oxygen and 0.1 mol of magnesium oxide. Dividing by 0.05 gives the ratio 2 : 1 : 2, so the equation is 2Mg + O2 → 2MgO.