In a chemical reaction, the theoretical yield is the amount of product that would form with one hundred percent conversion of the limiting reactant to the product.
Consider this example: Suppose 80 kernels are heated to make popcorn. Assuming all kernels will expand, the theoretical yield is 80 pieces of popcorn. However, if only 50 kernels pop, the actual yield is only 50 pieces. The actual yield is thus the amount of product that forms.
The ratio of the actual yield to the theoretical yield multiplied by one hundred gives the percent yield, which, in this case, is 62.5%
For many chemical reactions, the theoretical yield, which is based on stoichiometry, is greater than the yield that is actually obtained. Usually, some amount of the reactants is lost to side reactions, some of the product is lost to reversible reactions, or the product is difficult to collect without some loss.
Consider the burning of magnesium. When a piece of magnesium metal is ignited, it reacts with oxygen in the air to form magnesium oxide. The chemical reaction involves 2 moles of magnesium and one mole of oxygen to form 2 moles of magnesium oxide.
Supposing 63.4 grams of magnesium and 50.7 grams of oxygen, how much magnesium oxide could be produced?
To calculate the theoretical yield, first, the mass of each reactant is converted – using the molar masses – into the corresponding amounts in moles. Then, using mole ratios, the number of moles of the reactants — magnesium and oxygen — are converted to moles of product, magnesium oxide.
Since magnesium makes the least amount of product, it is the limiting reactant, and the theoretical yield is 2.6 moles. In terms of mass, the theoretical yield is 105 grams.
Once the reaction is complete, the actual yield of the product is 80.0 grams. Thus, the percent yield of the production of magnesium oxide is 76.2%.