By Kaylee Finn, Updated Aug 30, 2022
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Theoretical yield is a fundamental concept in chemistry that represents the maximum amount of product that can form when a reaction proceeds to completion. In practice, this means that the limiting reagent is fully consumed, and no further product can be produced from the remaining reactants. To determine the theoretical yield, you must first know the balanced chemical equation and the initial moles of each reactant.
Take, for example, the synthesis of water: H + O → H₂O. To balance this reaction you need two hydrogen atoms on the left to match the two in water, giving 2H + O → H₂O.
Assume you start with 5 mol of hydrogen and 3 mol of oxygen. The stoichiometric ratio required is 2 mol H for every 1 mol O. Using 3 mol of O would need 6 mol of H, but only 5 mol are available, so hydrogen is the limiting reagent.
Multiply the moles of the limiting reagent by the mole ratio between product and limiting reagent. For water, the ratio is 1 mol H₂O per 2 mol H. Thus, (1/2) × 5 mol H = 2.5 mol H₂O – the theoretical yield in moles.
To express the yield in grams, multiply the moles of product by its molar mass. For water, 2.5 mol × 18.02 g/mol = 45.05 g.
Balance the equation, find the limiting reagent, compute the product moles, then convert to grams using the product’s molar mass.
