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Theoretical Yield

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Theoretical Yield and Percent Yield

Can we save some money?

The world of pharmaceutical production is an expensive one. Many drugs have several steps in their synthesis and use costly chemicals. A great deal of research takes place to develop better ways to make drugs faster and more efficiently. Studying how much of a compound is produced in any given reaction is an important part of cost control.

Percent Yield

Chemical reactions in the real world don’t always go exactly as planned on paper. In the course of an experiment, many things will contribute to the formation of less product than would be predicted. Besides spills and other experimental errors, there are usually losses due to an incomplete reaction, undesirable side reactions, etc. Chemists need a measurement that indicates how successful a reaction has been. This measurement is called the percent yield.

To compute the percent yield, it is first necessary to determine how much of the product should be formed based on stoichiometry. This is called the theoretical yield , the maximum amount of product that could be formed from the given amounts of reactants. The actual yield is the amount of product that is actually formed when the reaction is carried out in the laboratory. The percent yield is the ratio of the actual yield to the theoretical yield, expressed as a percentage.

\text{Percent Yield} =\frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100\%

Percent yield is very important in the manufacture of products. Much time and money is spent improving the percent yield for chemical production. When complex chemicals are synthesized by many different reactions, one step with a low percent yield can quickly cause a large waste of reactants and unnecessary expense.

Typically, percent yields are understandably less than 100% because of the reasons indicated earlier. However, percent yields greater than 100% are possible if the measured product of the reaction contains impurities that cause its mass to be greater than it actually would be if the product was pure. When a chemist synthesizes a desired chemical, he or she is always careful to purify the products of the reaction.

Sample Problem: Calculating the Theoretical Yield and the Percent Yield

Potassium chlorate decomposes upon slight heating in the presence of a catalyst according to the reaction below:

2\text{KClO}_3(s) \rightarrow 2\text{KCl}(s)+3\text{O}_2(g)

In a certain experiment, 40.0 g KClO 3 is heated until it completely decomposes. What is the theoretical yield of oxygen gas? The experiment is performed and the oxygen gas is collected and its mass is found to be 14.9 g. What is the percent yield for the reaction?

Part 12.11A : First, we will calculate the theoretical yield based on the stoichiometry.

Step 1: List the known quantities and plan the problem.

Known

  • given: mass of KClO 3 = 40.0 g
  • molar mass KClO 3 = 122.55 g/mol
  • molar mass O 2 = 32.00 g/mol

Unknown

  • theoretical yield O 2 = ? g

Apply stoichiometry to convert from the mass of a reactant to the mass of a product:

\text{g KClO}_3 \rightarrow \text{mol KClO}_3 \rightarrow \text{mol O}_2 \rightarrow \text{g O}_2

Step 2: Solve.

40.0 \text{ g KClO}_3 \times \frac{1 \text{ mol KClO}_3}{122.55 \text{ g KClO}_3} \times \frac{3 \text{ mol O}_2}{2 \text{ mol KClO}_3} \times \frac{32.00 \text{ g O}_2}{1 \text{ mol O}_2}=15.7 \text{ g O}_2

The theoretical yield of O 2 is 15.7 g.

Step 3: Think about your result.

The mass of oxygen gas must be less than the 40.0 g of potassium chlorate that was decomposed.

Part 12.11B Now we use the actual yield and the theoretical yield to calculate the percent yield.

Step 1: List the known quantities and plan the problem.

Known

  • Actual yield = 14.9 g
  • Theoretical yield = 15.7 g (from Part 12.11A)

Unknown

  • Percent yield = ? %

\text{Percent Yield}=\frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100 \%

Use the percent yield equation above.

Step 2: Solve.

\text{Percent Yield}=\frac{14.9 \text{ g}}{15.7 \text{ g}} \times 100=94.9\%

Step 3: Think about your result.

Since the actual yield is slightly less than the theoretical yield, the percent yield is just under 100%.

Summary

  • Theoretical yield is calculated based on the stoichiometry of the chemical equation.
  • The actual yield is experimentally determined.
  • The percent yield is determined by calculating the ratio of actual yield/theoretical yield.

Practice

Work the problems found on the link below:

http://science.widener.edu/svb/tutorial/percentyieldcsn7.html

Review

  1. What do we need in order to calculate theoretical yield?
  2. If I spill some of the product before I weigh it, how will that affect the actual yield?
  3. How will spilling some of the product affect the percent yield?
  4. I make a product and weigh it before it is dry. How will that affect the actual yield?

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