Percentage Yield - Understanding Efficiency in Chemical Reactions
What Is Percentage Yield?
Percentage yield
In a chemical reaction, the percentage yield compares the actual amount of product obtained (experimental yield) to the maximum amount predicted by stoichiometry (theoretical yield).
It’s calculated using the formula:
$$\text{Percentage Yield} = \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100$$
- Actual Yield: The measured amount of product obtained from the reaction.
- Theoretical Yield: The maximum amount of product expected based on stoichiometry, assuming the reaction goes to completion without any losses.
If your theoretical yield is 10.0 g, but you only isolate 8.5 g of product, the percentage yield is:
$$\text{Percentage Yield} = \frac{8.5}{10.0} \times 100 = 85\%$$
Tip- Percentage yield can never exceed 100%, as this would violate the law of conservation of mass.
- If you calculate a yield over 100%, it’s likely due to measurement errors or impurities in the product.
Factors Affecting Yield
- Why don’t reactions always achieve 100% yield?
- Several factors can reduce the actual yield:
1. Loss of Product During Transfer or Purification
In laboratory experiments, product can be lost during steps like filtration, decanting, or transferring between containers.
ExampleSome solid might remain on the filter paper or dissolve in a washing solvent.
Common MistakeMany students forget to account for product losses during purification steps, leading to unrealistic expectations of 100% yield.
2. Side Reactions
- Chemical reactions don’t always proceed exactly as planned.
- Other reactions may occur simultaneously, consuming reactants and forming undesired by-products.
In the synthesis of aspirin, some salicylic acid might react with water instead of ethanoic anhydride, reducing the yield of aspirin.
NoteSide reactions are more common in complex organic syntheses, where multiple pathways are possible.
