A balanced chemical equation carries a wealth of information qualitative as well as quantitative. Quantitatively a balanced chemical equation specifies numerical relationship among the quantities of its reactants and products.
4Fe(s) + 3O2(g) → 2Fe2O3(s)
The reaction tells that 4 atoms of iron react with 3 molecules of oxygen to form 2 formula units of iron oxide.
Avogadro number of elementary entities like atoms, molecules, ions or formula units of a substance constitute one mole of it.
4 mol of Fe react with 3 mol of O2 and produce 2 mol of Fe2O3.
The mole relationship can be converted into mass relationship by using the fact that mass of one mole of any substance is equal to its molar mass which can be calculated from its formula with the help of relative atomic masses of its constituent elements.
Thus, 223.2 g iron would react with 96 g oxygen and produce 319.2 g iron oxide.
Substances which react with each other are not present in exactly the same proportion in a reaction mixture as stated by a balanced chemical equation. For example, if 2 mol each of hydrogen and oxygen are mixed and a spark is passed through the mixture, water is formed, according to the equation
2H2 + O2 → 2H2O
Here, 2 mol of hydrogen react with only 1 mol of oxygen, and 1 mol of oxygen therefore remains unreacted. In this example hydrogen is said to be the limiting reagent or reactant because its amount becomes zero and the reaction therefore stops before the other reactant (oxygen) is used up. The amount of hydrogen present initially limits the amount of product that is formed.