Percentage Yield & Percentage Purity

The quantity of product that is calculated to be produced when the entire limiting reactant is used up is called the theoretical yield.

The quantity of product that is actually produced through the reaction is called the actual yield.

Percentage yield can be calculated by:

$$\text{% yield} = \frac{\text{Actual yield}}{\text{Theoretical yield}} \times 100 \%$$

Percentage yield cannot be more than 100% as the actual yield is always less than the theoretical yield.


For a reaction: $2\text{H}_{2} + \text{O}_{2} \rightarrow 2 \text{H}_{2}\text{O}$. Consider the case where 5 g of $\text{H}_{2}$ reacted with 50 g of $\text{O}_{2}$. The actual yield is found to be 40 g. Calculate the theoretical yield and percentage yield.

  • No. of moles of $\text{H}_{2}$ present: $\frac{5}{2} = 2.5 \,  \text{moles}$
  • No. of moles of $\text{O}_{2}$ present: $\frac{50}{32} = 1.56 \, \text{moles}$
  • Since 2.5 moles of $\text{H}_{2}$ reacts with 1.25 moles of $\text{O}_{2}$, there is an excess of $\text{O}_{2}$. $\text{H}_{2}$ is the limiting reactant.
  • 2.5 moles of $\text{H}_{2}$ will yield 2.5 moles of $\text{H}_{2}\text{O}$, which equates to a theoretical yield of 45 g of $\text{H}_{2}\text{O}$.
  • Percentage yield: $\frac{40}{45} \times 100 \% = 88.9 \%$

Percentage Purity

Percentage purity indicates the amount of pure and impure substance present in a sample.

The percentage purity can be calculated by:

$$\text{% purity} = \frac{\text{Mass of pure substance in sample}}{\text{Mass of sample}} \times 100 \%$$


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