Percentage Yield (AQA GCSE Chemistry)
Revision Note
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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 Tips and Tricks
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:
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 Tips and Tricks
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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