The numbers in front of the formulae in a balanced equation show the mole ratio of the substances. This is the stoichiometry of the equation. You can deduce it from measured masses. Convert each mass to moles by dividing by its relative formula mass, then divide through to find the simplest whole-number ratio. For example, 4.8 g of magnesium (24 g per mol) is 0.20 mol, 3.2 g of oxygen (O2, 32 g per mol) is 0.10 mol, and the 8.0 g of magnesium oxide formed (40 g per mol) is 0.20 mol. The ratio Mg : O2 : MgO is 0.20 : 0.10 : 0.20, which is 2 : 1 : 2. The equation is 2Mg + O2 → 2MgO.
In practice one reactant often runs out first. This is the limiting reactant. When it is used up the reaction stops, so the amount of product depends on it. Any reactant left over is in excess.
To decide which reactant is limiting, divide the moles of each reactant by its number in the equation. The smallest answer is the limiting reactant. If 0.20 mol of magnesium is mixed with 0.20 mol of oxygen, magnesium gives 0.20 ÷ 2 = 0.10 and oxygen gives 0.20 ÷ 1 = 0.20. Magnesium is limiting. The equation needs only 0.10 mol of oxygen, so the rest of the oxygen is in excess. The amount of product is 0.20 mol of MgO, which is set by the magnesium.