How to Calculate the Mass of Reactants


Chemical equations detail the state before and after a chemical reaction has occurred. A large portion of chemical reactions proceed to completion if there are enough reactants to convert the reactants to products. The chemical equation details the quantities of reactants required to produce the products measured in moles. Converting the number of moles to the number of grams of each of the reactants helps determine how much reactant, by weight, to use for complete reaction to occur.

Things You'll Need

  • Balanced chemical equation
  • Periodic table
  • Calculator
  • List the reactants for the chemical equation in which you have interest. These are all the chemicals present on the left hand side of the equation.

  • Calculate the molecular weight of each of the reactants. Add together the atomic weight of each atom in each reactant. For example, for the chemical reaction that involves Zn and HCl, calculate the molecular weight of both reactants. Zn is an element so the molecular weight is the same as the atomic weight of Zn, which is 65.39 g/mol. The molecular weight of HCl is equal to the addition of the atomic weight of hydrogen and the atomic weight of chlorine, resulting in 1.008 + 35.453 = 36.461 g/mol.

  • Calculate the weight of the reactants using the molecular weight. Find the number of moles of each reactant used in the reaction. Calculate the weight by multiplying the molecular weight by the number of moles of the reactant. Continuing the example, assume that you used stoichiometric amounts of the reactants. Based on the equation, there is one mole of Zn and two moles of HCl. The weight of Zn used is (molecular weight) (number of moles) = (65.39) (1) = 65.39 grams of zinc. The weight of HCl used is (36.461) * (2) = 72.922 grams.


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