Every balanced chemical equation shows how many particles of each substance take part in a reaction. Because a mole is just a fixed count of particles, the coefficients in the equation also tell you the ratio of moles. For example, in the equation N2 + 3H2 gives 2NH3, the coefficients 1, 3, and 2 mean that one mole of nitrogen reacts with three moles of hydrogen to make two moles of ammonia.
These ratios become conversion factors. From the ammonia example, you can write ratios such as 3 moles H2 per 1 mole N2, or 2 moles NH3 per 1 mole N2. To find how much of one substance reacts with or forms from another, you multiply the known amount in moles by the correct mole ratio so that the unwanted unit cancels.
Mole ratios sit at the center of most stoichiometry problems. A common three-step path is to convert a given mass to moles using molar mass, apply the mole ratio to switch from one substance to another, and then convert back to mass. The mole ratio is the only step that carries information from the balanced equation, so getting the coefficients right is essential.
Mole ratios are also used to find the limiting reactant, the substance that runs out first and controls how much product forms. By comparing the actual amounts of reactants to the ratio required by the equation, you can see which reactant is in short supply. The limiting reactant then sets the theoretical yield, the maximum amount of product the reaction can make.