How to Find the Limiting Reactant in Stoichiometry

By Sean Lancaster
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The language of chemistry is the chemical equation. The chemical equation defines what occurs during a given chemical reaction. Stoichiometry is the term used to describe the ratios of reactants that interact to produce products. According to the first law of physics, you can neither create nor destroy matter. The reactants of a chemical reagent can only make products according to the chemical equation until you use up one of the reactants, then the reaction stops. The limiting reactant is the reactant present in the least amount. The chemical equation expresses the amount of reactants and products in moles not weight. A mole describes a specific number of atoms or molecules used in chemical reactions equals 6.02 X 10^23 particles.

Balance the chemical equation of interest. Simple chemical equations must balance the atoms in the reactants with the atoms in the products. The electrical charge on the reactant side of the equation must equal the electrical charge on the products side of the equation. For example, assume that the reaction is: Na + Cl2 --> NaCl. To balance the equation, the number of sodium (Na) atom and chloride (Cl2) atoms on the reactant side must equal the number on the product side. To make the equation balance, add one sodium atom to the reactant side of the equation and change the number of NaCl to two. The balanced equation is 2 Na + Cl2 --> 2 NaCl.

Convert the number of grams of reactants to the number of moles of reactants by dividing the weight of reactant in grams by the atomic or molecular weight of the reactants. Continuing the example, atomic weight of sodium is 22.99 grams and the atomic weight of chlorine is 35.45 grams. Since chlorine exists as a diatomic molecule, the molecular weight is 70.90 grams. Assume you have 1.5 grams of sodium and 3.55 grams of Cl2. Divide the weight of each of the reactants by their atomic or molecular weight to obtain the number of moles with which you begin the reaction. Sodium, (1.5) / (22.99) = 0.0625 moles and chlorine, (3.55) / (70.90) = 0.0473 moles.

Compare the ratios of the reactants to the stoichiometry of the balanced equation. Divide the number of moles of each reactant by the number of atoms or molecules required for the reaction. Continuing the example, sodium is (0.0625) / 2 = .0313 and chlorine is (0.0473) / 1 = 0.0473.

Examine the stoichiometry of the chemical equation and determine the amount of the other reactants required to completely exhaust the amount of a single reactant. The reactant with the least amount to satisfy the balanced equation is the limiting reactant. Following the example, based on the balanced equation to use all the Cl2 available (0.0473 moles), you would need 0.0946 moles of Na. To exhaust the amount of sodium (0.0625 moles) you would need to have 0.0313 moles of Cl2. This evaluation indicates that Cl2 is present in excess, so Na is the limiting reactant.

About the Author

Sean Lancaster has been a freelance writer since 2007. He has written for Writers Research Group, Alexis Writing and the Lebanon Chamber of Commerce. Lancaster holds a Doctor of Philosophy in chemistry from the University of Washington.