4.3 Percent Yield

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Learning Objectives

Differentiate between actual yield, theoretical yield, and percent yield
Calculate percent yield

Percent Yield

There are many reasons why the actual yield of a chemical reaction may be less than the theoretical yield, and these will be taken up during later chapters of the course. Here are some reasons.

Equilibria between products and reactants, where the limiting reagent is not completely consumed (chapter 15)
Kinetics, simply speaking, the reaction may be very slow and not over (chapter 14).
Formation of Intermediates. A chemical equation does not represent the mechanism (chapter 14), and there are often intermediate chemical species that are not completely consumed. This is exemplified in the following problem where phosphoric acid is formed from phosphorous, oxygen and water.

Consider the reaction:
4P + 5O₂ + 6H₂O ---> 4H₃PO4

Which could result from the addition of the following two equations, where P₄O₁₀is an intermediate

4P + 5O₂ -----> P₄O₁₀
P₄O₁₀ + 6H₂O ---> 4H₃PO₄

If phosphorous is the limiting reagent it could be completely consumed, but the percent yield could still be less than 100% because of the existence of an intermediate, P₄O₁₀.
Alternative chemical pathways. Mixing chemicals together can result in more than one type of reaction, and if there are parallel reactions competing for reactants the actual yield of a desired product will be decreased by the formation of other compounds.
Experimental Error - even with reactions that may go to completion, experimenter technique may impact that amount of product collected as well as the presence of impurities in the collection process.

Percent Yield is defined as the actual yield divided by the theoretical yield times 100.

\[\text{Percent Yield} = \left ( \dfrac{\text{Actual Yield}}{\text{Theoretical Yield}} \right ) \times 100\%\]

Percent Yield Problem

Phosphoric Acid can be synthesized from Phosphorous, Oxygen and water according to the following equations

\[4P + 5O_2 \rightarrow P_4O_{10}\]

\[P_4O_{10} + 6H_2O \rightarrow 4H_3PO_4\]

What is the percent yield if 50.3 g of phosphoric acid is created from 20.0 g phosphorous, 15.0 g of water and 30.0 g of oxygen?

4:40 min YouTube solving for the percent yield, this video shows the quick way to find limiting reagent.

Click the following link to complete more practice with feedback and hints on calculating percent yield.

http://chemcollective.org/activities/tutorials/stoich/percentyield

Click the following link for more practice from ChemTours with feedback: Click Ch 3, Limiting Reactant.

https://wwnorton.com/college/chemistry/chem4/chemtours.aspx

Practice Worksheets:

Percent Yield Worksheet

Percent Yield Worksheet Key