Chemistry only
Percentage yield
You might spend a lot of time working out a method to make a product that you hope to sell for
vast profit, let's call this money making substance; substance C. Your method is to use the reactants A and B to make your
desired product C.
A + B → C
Looking at this equation it would appear at first glance to be a good method to make substance C,
there are no other products so the atom economy is 100% since all of the reactants A and B form one single product.
Now you probably already know that you can use mole type calculations to work out the theoretical yield of a reaction, that is the mass of the product you expect
to make based on how much of the reactants, A and B you intend to measure out and use.
While the actually yield is the mass of the product that you actually manage to make,
and that you can bottle up and
sell. In an ideal world the theoretical yield, the calculated yield and actual yields would be the
same. You might expect that you would get 100% of the product you expect, unfortunately in the real world actual yields are very different from the expected calculated theoretical yield and are very rarely if ever 100%.
Why you never get what you expect!
There are many reasons why the actual yields obtained from carrying out a practical activity are never 100%, that is you never manage to make all the product you expect, no matter how carefully you carry out the experiment; these include:
- One of more of your reactants is not 100% pure. This means that if you weighed
out 20g of it, then there might only be 19g of the actual reactant you need, the other 1g would
be a contaminant. This would mean you make less product than you expect.
- The method chosen by you to make the desired product could be a reversible reaction
and not one which goes to completion. This would mean that an equilibrium mixture of
reactants and products would be obtained and the amount of product you expect
to get would be much less than you hoped for simply because some of the products would be turning back into reactants in the equilibrium mixture.
- There may be side reactions that you did not expect. This would mean some of your reactants are
turned into an unwanted product. This again would reduce the actual yield of desired product.
- When products are transferred from one container to another some substances are always left behind.
The more steps and the more transfers that take place the more substances will be lost and the actual
yield will go down. So it is crucial to design a method which involves as few steps as possible.
- Some gases, liquids or solids could escape into the air, this would mean you lose some reactant or
product and again your actual yield will go down.
Percentage yield
To calculate the percentage yield of a reaction use the formula below:
Example calculations
In an experiment Joe made 7g of substance A. He calculated that the maximum theoretical mass of product should
have been able to make was 10g. What is the percentage yield of this reaction?
Simply use the equation given above to calculate the percentage yield,.
- Mass of desired product = 7g
- Theoretical (calculated) mass was 10g.
Simply substitute these numbers into the equation to get the percentage yield.
Percentage yield = (7g/10g) x 100% = 70% yield.
Example 2:
2.5g of new drug is made but the scientists were expecting a maximum theoretical yieldof 15g. What is the percentage yield for this reaction?
- Actual yield of product= 2.5g
- Theoretical yield that could be obtained is 15g.
So simply substitute these numbers into the formula to give:
Percentage yield = (2.5g/15g) x 100% = 17% yield.
Practice questions
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