The percent yield is the actual amount of reaction product obtained from a reaction compared to the theoretical maximum amount that could be obtained. A stoichiometry calculation shows how much of each reaction product could be obtained in a particular reaction, given the amounts of each reactant used. If too little of one reactant is provided at the beginning of the reaction, the actual yield will go down and some reactants may be left over. Several other factors may reduce the percent yield as well.
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The percent yield is given by the actual yield of a particular reaction product divided by the maximum theoretical yield, both in grams, multiplied by 100. Usually the percent yield is less than 100 percent due to inaccuracies in the measurements, the reaction not running to completion or a limited availability of one of the reactants.
The Theoretical Yield
A stoichiometry calculation can determine the theoretical yield for a particular reaction. The reaction equation is first balanced, and then the reactants and reaction products are expressed in moles of each substance. A gram-per-mole conversion gives the weights.
For example, hydrogen gas and oxygen gas react to form water. The unbalanced equation is H2 + O2 = H2O, with the equation showing that the hydrogen and oxygen gases exist as two-atom molecules. The equation is unbalanced because there is only one oxygen atom on the right and two on the left. The corresponding balanced equation is 2H2 + O2 = 2H2O.
Expressing the balanced equation in moles means that two moles of hydrogen gas and one mole of oxygen gas react to form two moles of water. Converting to grams using the gram atomic weights from a periodic table of the elements gives the following weights: one mole of hydrogen gas: 2 g, one mole of oxygen gas: 32 g, and one mole of water: 18 g. According to the equation in grams, 4 grams of hydrogen react with 32 grams of oxygen to form 36 grams of water, which is the theoretical yield of this reaction.
The Percent Yield
In real reactions, the actual yield is usually lower than the theoretical yield. The weights of the reactants may not be exactly balanced so the reaction products are less. The reaction may not use all of one of the reactants because, as a gas, some is lost into the air. Sometimes the reactants have impurities that cause the reaction to stop, and sometimes the reaction product gets in the way of a complete reaction. For all these reasons, less product is produced than the theoretical maximum.
When the actual weight of the reaction product is less than the theoretical value, the result can be expressed as a percentage. The actual weight divided by the theoretical weight multiplied by 100 gives the percent yield of the reaction.
In the reaction of hydrogen and oxygen, the theoretical weight of the reaction product for 4 grams of hydrogen and 32 grams of oxygen was 36 grams of water. But in a real reaction, if 4 grams of hydrogen burns in air, the amount of water produced might only be 27 grams. Dividing the actual value of 27 grams by the theoretical value of 36 grams and multiplying by 100 gives 75 percent, the percent yield for this reaction.