Percentage Yield Calculations

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:

$percentage yield = \frac{actual yield}{theoretical yield} \times 100$

- 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 balanced symbol equation is:

Zn (s) + CuSO₄ (aq) → ZnSO₄ (aq) + Cu (s)

Step 2: Calculate the amount of zinc reacted in moles

number of moles = \frac{6.5 g}{65.4 g {mol}^{- 1}} = 0.10 mol

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

mass = 0.10 mol x 63.55 g mol^-1

mass = 6.4 g (2 sig figs)

Step 5: Calculate the percentage yield of copper

percentage yield = \frac{4.8 g}{6.4 g} \times 100 = 75 %

Percentage Yield Calculations (SL IB Chemistry)

Revision Note