Percentage Yield (WJEC GCSE Science (Double Award)): Revision Note

How to Calculate Percentage Yield

Percentage Yield

• Yield is the term used to describe the amount of product you get from a reaction

• In practice, you never get 100% yield in a chemical process for several reasons

• These include:

  • Some reactants may be left behind in the equipment

  • The reaction may be reversible and in these reactions a high yield is never possible as the products are continually turning back into the reactants

  • Some products may also be lost during separation and purification stages such as filtration or distillation

  • There may be side reactions occurring where a substance reacts with a gas in the air or an impurity in one of the reactants

  • Products can also be lost during transfer from one container to another

Actual & Theoretical Yield

• The actual yield is the recorded amount of product obtained

• The theoretical yield is the amount of product that would be obtained under perfect practical and chemical conditions

• It is calculated from the balanced equation and the reacting masses

• The percentage yield compares the actual yield to the theoretical yield

• For economic reasons, the objective of every chemical producing company is to have as high a percentage yield as possible to increase profits and reduce costs and waste

• To calculate percentage yield the following equation is used: