Percent Yield: From Theoretical Yield to the Number in Your Lab Report
Percent yield is the actual mass of product you isolated divided by the theoretical mass the balanced equation predicts, times 100. It measures how much of the possible product survived the reaction and the workup.
Getting the theoretical yield
- Balance the equation and identify the limiting reagent.
- Convert its mass to moles: moles = mass ÷ molar mass.
- Apply the mole ratio from the equation to get moles of product.
- Multiply by the product's molar mass for the theoretical yield in grams.
Worked example: aspirin
Start with 2.00 g of salicylic acid (138.12 g/mol), which is 0.01448 mol. The synthesis with acetic anhydride runs 1:1, so 0.01448 mol of aspirin is possible; at 180.16 g/mol that is a theoretical 2.61 g. Isolating 2.09 g of dry product gives a percent yield of 80.1%.
Why yields fall short — and why they must not exceed 100%
- Incomplete reaction, competing side reactions and reversible equilibria.
- Product lost in transfers, on filter paper and dissolved in the recrystallisation liquor.
- A yield above 100% always means impurity: residual solvent, water or unreacted starting material.
- Dry to constant mass before weighing, and check the melting point for purity.
Key takeaways — Percent Yield Calculator
- Percent yield = actual ÷ theoretical × 100.
- Theoretical yield comes from the limiting reagent and the mole ratio.
- Over 100% is a purity problem, not a good result.