Reaction Yields
Percentage yield
- In a lot of reactions, not all reactants react to form products which can be due to several factors:
- Other reactions take place simultaneously
- The reaction does not go to completion
- Products are lost during separation and purification
- The percentage yield shows how much of a particular product you get from the reactants compared to the maximum theoretical amount that you can get:
- The actual yield is the number of moles or mass of product obtained experimentally
- The theoretical yield is the number of moles or mass obtained by a reacting mass calculation
Worked example
In an experiment to displace copper from copper(II)sulfate, 6.5 g of zinc was added to an excess of copper(II)sulfate solution.The resulting copper was filtered off, washed and dried.The mass of copper obtained was 4.8 g.Calculate the percentage yield of copper.
Answer:
Step 1: The symbol equation is:
Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)
Step 2: Calculate the amount of zinc reacted in moles
Step 3: Calculate the maximum amount of copper that could be formed from the molar ratio:
Since the ratio of Zn(s) to Cu(s) is 1:1 a maximum of 0.10 moles can be produced
Step 4: Calculate the maximum mass of copper that could be formed (theoretical yield)
mass = mol x M
= 0.10 mol x 63.55 g mol-1
= 6.4 g (2 sig figs)Step 5: Calculate the percentage yield of copper
