Chemistry / Unit 8: Stoichiometry
8.4 Theoretical and Percent Yield
Notes
The theoretical yield is the amount of product calculated from the limiting reactant. The actual yield is what you really get in the lab.
- $\displaystyle \text{percent yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100\%$.
- Actual yield is usually less than theoretical because of side reactions, incomplete reactions and loss during transfer.
- A percent yield over 100% usually means the product is still wet or impure.
- Example: theoretical 20.0 g, actual 17.0 g. Percent yield $\displaystyle = \frac{17.0}{20.0} \times 100\% = 85.0\%$.
Practice
Heating 50.0 g of $\text{CaCO}_3$ should produce 28.0 g of CaO. A student collects 25.2 g. What is the percent yield?
$\displaystyle \frac{25.2}{28.0} \times 100\% = 90.0\%$.
A reaction has a theoretical yield of 40.0 g and a typical percent yield of 75.0%. What actual yield should you expect?
$40.0 \times 0.750 = 30.0$ g.
A student reports a percent yield of 112%. What is the most likely reason?
Leftover water adds mass, making the actual yield seem too large.
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