Determine the number of moles present for each species in the reaction. This is determined by the coefficients of each species in the balanced chemical equation. For example, in the reaction of hydrogen and oxygen to produce water, 2H2 + O2 = 2H2O. There are 2, 1 and 2 moles of hydrogen, oxygen and water, respectively.
Multiply the number of moles for the two species of interest by their molecular mass per mole. Continuing from above, 2 moles of hydrogen x 1 gram/mole (the molecular mass of hydrogen) = 2 grams, and 2 moles of H2O times its molecular mass, which is 18 grams/mole = 2 moles x 18 grams/mole = 36 grams.
Divide the reactant mass, hydrogen in this example, by the product mass, water, to achieve a mass ratio: 2 grams / 36 grams simplifies to 1/18th. This resulting quotient is the mass ratio of the two species.