gases heading

Chemistry only

Percentage yield

good idea 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 + BC
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!

Chemistry student carrying out a practical activity in the lab 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:

Percentage yield

To calculate the percentage yield of a reaction use the formula below: formula for % yield

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,.

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? So simply substitute these numbers into the formula to give:
Percentage yield = (2.5g/15g) x 100% = 17% yield.

Practice questions


Check your understanding - questions on percentage yields

Check your understanding - additional questions on percentage yields