Percent Yield
In a perfect world, every reaction would produce exactly the amount of product predicted by stoichiometry. In reality, they never do. Some product sticks to the glassware. Some reactant undergoes a side reaction. Some product decomposes before you can collect it. The gap between what you should get and what you actually get is measured by percent yield.
Three Types of Yield
| Term | Definition |
|---|---|
| Theoretical yield | Maximum product possible, calculated from the limiting reagent (the “perfect” amount) |
| Actual yield | Amount of product you actually isolate in the lab (always measured experimentally) |
| Percent yield | (Actual yield / Theoretical yield) x 100% |
Why Percent Yield is Always Less Than 100%
Several factors reduce actual yield:
- Incomplete reactions - not all reactant converts to product (equilibrium is reached before completion)
- Side reactions - reactants form unwanted byproducts
- Mechanical losses - product left on glassware, lost during filtration or transfer
- Purification losses - product lost during recrystallization, distillation, or chromatography
- Decomposition - some product breaks down before collection
Worked Example
Problem: A student reacts 5.00 g of zinc with excess hydrochloric acid. The balanced equation is:
Zn + 2 HCl → ZnCl₂ + H₂
The student collects 0.125 g of H₂. What is the percent yield?
Step 1: Calculate theoretical yield.
- Moles of Zn = 5.00 g / 65.38 g/mol = 0.0765 mol
- HCl is in excess, so Zn is the limiting reagent
- Mole ratio: 1 mol Zn → 1 mol H₂
- Theoretical moles of H₂ = 0.0765 mol
- Theoretical yield = 0.0765 mol x 2.016 g/mol = 0.154 g
Step 2: Calculate percent yield.
- % yield = (0.125 g / 0.154 g) x 100% = 81.2%
Working Backward from Percent Yield
The MCAT might give you a percent yield and ask you to find how much reactant you need to produce a desired amount of product.
Example: You need 50.0 g of product. The reaction has a 75% yield. How many grams of theoretical yield do you need?
Theoretical yield = actual yield / (% yield / 100) = 50.0 / 0.75 = 66.7 g
Then calculate the reactant needed for 66.7 g of product using standard stoichiometry.
What Percent Yield Greater Than 100% Means
If your calculated percent yield exceeds 100%, it does not mean you created matter from nothing. It means your product is impure. Common causes:
- Product was not fully dried (residual solvent adds mass)
- Product is contaminated with unreacted starting material or byproducts
- Product absorbed moisture from the air
This is a favorite MCAT distractor. If a question asks “a student obtained a 110% yield, what is the most likely explanation?” the answer is always contamination or incomplete drying - never that the reaction was unusually efficient.