By Emma Francesca • Updated Aug 30, 2022
In a chemical reaction, reactants combine in precise stoichiometric ratios to form products. Under ideal laboratory conditions, you can predict exactly how much product will emerge from a given amount of reactant. That predicted quantity is called the theoretical yield. Calculating it accurately requires knowing the amounts of reactants and products actually used and identifying the limiting reactant.
Begin with a correctly balanced chemical equation. This ensures the mole ratios between reactants and products are accurate.
Use a reliable periodic table to sum the atomic weights of all atoms in each species. These values are your molar masses.
From the balanced equation, note the mole ratios. For example, if 1 mol of reactant A produces 2 mol of product B, the ratio is 1:2.
If the quantities you have match those in the balanced equation, the theoretical yield equals the product amount shown in the equation. Convert that value to grams by multiplying moles by the product’s molar mass.
When you start with masses in grams, divide each by its molar mass to obtain moles.
Using the stoichiometric ratios from Step 3, compare the moles you have (from Step 5) to the moles required. The reactant that runs out first limits the reaction.
Apply the mole ratio between the limiting reactant and the desired product. For instance, if 2 mol of the limiting reactant produce 3 mol of product, then 1 mol of the limiting reactant yields 1.5 mol of product.
Multiply the theoretical moles of product by its molar mass to obtain the yield in grams.
If a reaction involves a single reactant, that reactant is automatically the limiting reactant.
