How much did you make?

Section 4 of 4

Chemists weigh what they make and compare it with the most they could have made. The answer, as a percentage, is the yield.

The most you can make

Atoms have different masses. On the scale chemists use, a hydrogen atom has a mass of 1, a carbon atom 12 and an oxygen atom 16. Add up the atoms in a molecule and you have its mass on the same scale:

  • salicylic acid, C₇H₆O₃: 7 × 12 + 6 × 1 + 3 × 16 = 138
  • aspirin, C₉H₈O₄: 9 × 12 + 8 × 1 + 4 × 16 = 180

Each salicylic acid molecule becomes one aspirin molecule. So 138 g of salicylic acid can make at most 180 g of aspirin, and 1.38 g can make at most 1.80 g. Chemists add more ethanoic anhydride than they need, so salicylic acid is the one that runs out first, and it sets the limit.

Working out the yield of a batch of aspirin
salicylic acid used1.38 g
most aspirin it can make1.38 × 180 ÷ 138 = 1.80 g
dry aspirin weighed1.35 g
yield1.35 ÷ 1.80 × 100 = 75%

Where the rest goes

A yield of 100% is out of reach, because aspirin goes missing at every stage:

  • some salicylic acid is still unreacted when the warming stops
  • water splits a little aspirin back into salicylic acid
  • in a side reaction, the -⁠COOH of one salicylic acid molecule joins the -⁠OH of another, making a different ester
  • some aspirin stays dissolved in the cold liquid and runs through the filter, and some sticks to the flask and the filter paper
  • recrystallising loses a little more, the price of a purer product

Step after step

Most medicines take several steps to make, and the yields multiply. Three steps that each give 80% leave 0.8 × 0.8 × 0.8 = 0.512 of the starting material as product: about 51%.

Yield across three steps of 80% each A bar chart of how much of the starting material is left as product: start 100%, after step 1 80%, after step 2 64%, after step 3 51%. Each bar is 80% of the one above it, and the dashed outlines show the full 100%. start 100% after step 1 80% after step 2 64% after step 3 51% each step keeps 80%
Three steps of 80% each. Each step keeps 80% of what it starts with, so 100% falls to 80%, then 64%, then about 51%.

Ten steps of 90% each give about 35%. So chemists look for routes with fewer steps, and for reactions that give high yields at every one.

Quick check

A synthesis has two steps, and each step gives a 50% yield. What is the yield overall?

That's the end of lesson 24

In this lesson you found out:

  • How chemists plan a synthesis by working backwards from the target
  • How aspirin is made from salicylic acid in a lab
  • How chemists check that a product is pure
  • How to work out a yield, and why it falls with every step
Start Lesson 25: Greener chemistry All lessons