Percent yield
- The theoretical yield is the maximum amount of product, calculated by stoichiometry from the limiting reactant.
- The actual yield is the amount really obtained in the lab.
percent yield = (actual yield ÷ theoretical yield) × 100
The actual yield is usually lower. Some product sticks to glassware, some reactant does not react, and side reactions make other products.
Worked example: from the two yields
A reaction should produce 25.0 g of product. A student collects 21.5 g.
percent yield = (21.5 g ÷ 25.0 g) × 100 = 86.0%
Worked example: starting from a reactant
2 Mg + O2 → 2 MgO. Burning 12.15 g of Mg gives 18.1 g of MgO. What is the percent yield?
Theoretical yield: 12.15 g ÷ 24.305 g/mol = 0.4999 mol of Mg, which makes 0.4999 mol of MgO. Then 0.4999 mol × 40.304 g/mol = 20.15 g.
percent yield = (18.1 g ÷ 20.15 g) × 100 = 89.8%
Common mistake
Putting the theoretical yield on top. The actual yield goes on top. A result over 100% usually means the product is wet or impure, or the fraction is upside down.