PES Skill Sheets.book - Capital High School

Name: Date:
14.2 Percent Yield
You can predict the amount of product to expect from a reaction if you know how much reactant you started with.
For example, if you start out with one mole of limiting reactant, you can expect to produce one mole of product.
In real-world chemical reactions, the actual amount of product is usually less than the predicted amount. This is
due to experimental error and other factors (such as the fact that some product is difficult to collect and measure).
The amount of product you expect to produce is called the predicted yield. The amount of product that you are
able to measure after the reaction is called the actual yield. The percent yield is the actual yield divided by the
predicted yield and then multiplied by 100.
Actual yield
Percent yield =
----------------------------------- × 100
Predicted yield
The percent yield can provide information about how carefully the experiment was performed. If a percent yield
is low, chemical engineers look for sources of error. Manufacturers of chemical products try to maximize their
percent yield so that they can get the maximum amount of product to sell from the reactants that they purchased.
I
• In the reaction below, potassium and water are combined in a chemical reaction that produces potassium
hydroxide and hydrogen gas. If two moles of potassium (the limiting reactant) are used, what is the predicted
yield of potassium hydroxide (KOH) in grams?
2K + 2H 2O → 2KOH + H 2
1. Looking for: Predicted yield of KOH in grams
2. Given: Two moles of the limiting reactant (K) are used
3. Relationships: Two moles of limiting reactant should produce two moles of product. Two moles of
the product will have a mass twice its molar mass.
4. Solution: Molar mass of KOH = atomic mass of K + atomic mass of O + atomic mass of H
From periodic table, molar mass of KOH = 39.10 + 16.00 + 1.01 = 56.11 grams
Because we started with two moles of limiting reactant, we should end up with two
moles, or 112.22 grams, of KOH.
• If the actual yield of KOH was 102.5 grams, what was the percent yield for this reaction?
1. Looking for: Percent yield of KOH
2. Given: Actual yield = 102.5 g; predicted yield = 122.22 g
3. Relationships: Percent yield = actual yield ÷ predicted yield × 100
4. Solution: Percent yield = 102.5 g ÷ 112.22 g × 100 = 91.3%
14.2