Molar ratio is used to describe the proportional relationship between the amounts of substances involved in a chemical reaction. It comes from the coefficients in the balanced chemical equation and provides a conversion factor between the different reactants and products in stoichiometry.

The molar ratio is expressed as a ratio of the number of moles of one substance to the number of moles of another substance in a chemical reaction.

For example, in the reaction:     2 H2 + O2 → 2 H2O

The molar ratio between hydrogen gas (H2) and water (H2O) is 2:2, or simply 1:1. This means that for every 2 moles of hydrogen gas consumed, 2 moles of water are produced.