Moles – Percentage Purity and Yield

The yield is the amount of product you obtain from a reaction. Suppose we own a factory that makes fertilizers. We will want the highest yield possible, for the lowest cost.

If we are making medical drugs then the yield will still be important, but the purity of the product may be even more important. This is because the impurities may harm the people using the drugs.

 Finding the percentage yield

The formula for percentage yield is:

Aspirin is made from salicylic acid. 1 mole of salicylic acid gives 1 mole of aspirin. The chemical formula for salicylic acid is C7H6O3 and the chemical formula for aspirin is C9H8O4.

In an experiment, 100.0 g of salicylic acid yielded 121.2 g of aspirin. What was the percent yield?


1. Calculate the Mr (RMM = relative molecular mass) of the substances.

  • Ar : C = 12, H = 1, O = 16
  • So, Mr : salicylic acid = 138, aspirin = 180.

2. Convert  grams to moles for salicylic acid

  • 138 g of salicylic acid = 1 mole
  • So, 100 g = 100 ÷ 138 mole = 0.725 moles

3. Work out the calculated mass of the aspirin.

  • 1 mole of salicylic acid gives 1 mole of aspirin
  • So, 0.725 moles gives 0.725 moles of aspirin
  • 0.725 moles of aspirin = 0.725 × 180 g = 130.5 g
  • So, the calculated mass of the reaction is 130.5 g

4. Calculate the percent yield.

  • The actual mass obtained is 121.2 g
  • So, the percent yield = 121.2 ÷ 130.5 × 100% = 92.9%

Finding Percentage Purity

When we make something in a chemical reaction, and separate it from the final mixture, it will still have small amounts of other substances mixed with it. It will be impure.

The formula for percentage purity is:


The aspirin from the above experiment was not pure. 121.2 g of solid was obtained, but analysis showed that only 109.2g of it was aspirin. Calculate the percentage purity of the product.


Percentage purity = 109.2 ÷ 121.2 × 100% = 90.0%

Topic Headers for Revision

As usual, here’s a revision guide with page numbers from the textbooks. It’s better to work from the Curriculum Content, however and look stuff up in the textbooks as you need  to.  For everybody in year 11 plus early entry year 10 you need to download the pdf for 2012, for everyone else, download the pdf for 2013 or 2014.