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Percentage Yield (AQA GCSE Chemistry)

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Obtaining calculated masses: basics

  • 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

Examiner Tip

Although it’s very rare that they are equal, an efficient and well managed chemical process will produce an actual yield that is close to the theoretical yield.

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:

percentage space yield space equals space fraction numerator actual space yield over denominator theoretical space yield end fraction space cross times space 100

Worked example

Copper(II) sulfate may be prepared by the reaction of dilute sulfuric acid with 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%

Examiner Tip

  • 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

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    Stewart

    Author: Stewart

    Expertise: Chemistry Lead

    Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.