Why yields fall short
A balanced equation tells you the most product a reaction could give, the theoretical yield. In the lab you almost always get less: some product sticks to glassware, stays dissolved in the filtrate, is lost during recrystallization, or the reaction does not go to completion. The percent yield measures how close you came.
The formulas
- Moles of limiting reagent = mass ÷ molar mass
- Moles of product = moles of reagent × (product coefficient ÷ reagent coefficient)
- Theoretical yield = moles of product × molar mass of product
- Percent yield = actual yield ÷ theoretical yield × 100
Worked example: aspirin synthesis
Salicylic acid (C7H6O3, 138.12 g/mol) reacts with acetic anhydride to make aspirin (C9H8O4, 180.16 g/mol), one to one. Acetic anhydride is in excess, so salicylic acid is the limiting reagent.
- Starting mass: 2.00 g of salicylic acid, or 2.00 ÷ 138.12 = 14.5 mmol.
- The equation is 1:1, so at most 14.5 mmol of aspirin.
- Theoretical yield: 0.0145 × 180.16 = 2.61 g.
- You recover 2.10 g of dry crystals: percent yield = 2.10 ÷ 2.61 = 80.5%.
Finding the limiting reagent
When more than one reactant is measured out, divide the moles of each by its coefficient in the balanced equation. The smallest result is the limiting reagent, and it alone sets the theoretical yield. The others are in excess.
What is a good yield?
| Percent yield | Typical meaning |
|---|---|
| Above 100% | Product wet or impure |
| 90 to 100% | Excellent, rare after purification |
| 70 to 90% | Good for a student lab synthesis |
| 40 to 70% | Average; check transfers and recrystallization |
| Below 40% | Low; something went wrong |
For multi-step syntheses, yields multiply: five steps at 80% each give an overall 0.8⁵ = 33%.
Improving your yield
- Dry the product to constant mass before weighing; water adds weight and inflates the yield.
- Rinse glassware with a little cold solvent to recover crystals.
- Cool the solution fully before filtering, so less product stays dissolved.
- Do not over-recrystallize: each recrystallization improves purity but costs product.
- Check the reaction time and temperature in the procedure; an incomplete reaction is a common cause of low yields.