Calculate Percentage Yield (Edexcel IGCSE Chemistry)

Spec point — calculate percentage yield

How to calculate 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 and 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

Calculating percentage yield

• The percentage yield is a good way of measuring how successful a chemical process is
• There are often several methods of creating a compound and each method is called a reaction pathway
• Reaction pathways consist of a sequence of reactions which must occur to produce the required product
• Companies often investigate and try out different reaction pathways and these are then compared and evaluated so that a manufacturing process can be chosen
• The percentage yield of each pathway is a significant factor in this decision making process
• The equation to calculate the percentage yield is:

$\mathrm{percentage} \mathrm{yield}=\frac{\mathrm{actual} \mathrm{yield}}{\mathrm{theoretical} \mathrm{yield}}\times100$

Worked Example Copper(II) sulfate may be prepared by the reaction of dilute sulfuric acid on copper(II) oxide.

A student prepared 1.6 g of dry copper(II) sulfate crystals. Calculate the percentage yield if the theoretical yield is 2.0 g.

Answer:

• Actual yield of copper(II) sulfate = 1.6 g
• Percentage yield of copper(II) sulfate = (1.6 / 2.0) x 100
• Percentage yield = 80%

Exam Hint

• The actual yield and the percentage yield can be determined by experiment only

  • The theoretical yield can be calculated assuming there is 100% conversion of reactants to products.

• You are expected to remember the equation for percentage yield

• If you remember it incorrectly and get a percentage yield greater than 100%, then you have made an error!

  • The most common error is to divide the theoretical yield by the actual yield
  • In this case, you just need to swap the numbers around in your calculation