Percent Yield Formula
The percent yield formula compares actual product obtained to theoretical maximum.
Essential for chemistry lab calculations.
The Formula
Percent yield measures how efficient a chemical reaction is by comparing what you actually produced to the maximum amount that could theoretically be produced.
Variables
| Symbol | Meaning |
|---|---|
| Percent Yield | Efficiency of the reaction (expressed as a percentage, %) |
| Actual Yield | The amount of product actually obtained from the experiment (measured in grams or moles) |
| Theoretical Yield | The maximum possible amount of product calculated from stoichiometry (same units as actual yield) |
Understanding Yield Values
- 100% yield: All reactants converted to product. Rarely achieved in practice.
- Above 90%: Excellent yield. Common for simple, well-optimized reactions.
- 70-90%: Good yield. Typical for many laboratory reactions.
- Below 50%: Poor yield. May indicate side reactions, incomplete reaction, or loss during purification.
- Above 100%: Not possible in theory. Usually indicates impurities in the product or measurement error.
Example 1
A reaction should theoretically produce 25.0 g of aspirin. The student actually obtained 18.5 g. What is the percent yield?
Apply the formula: Percent Yield = (18.5 / 25.0) × 100%
Percent Yield = 74.0%
Example 2
A chemist needs 50 g of product. The reaction has an 80% yield. How much theoretical yield must the reaction be set up for?
Rearrange: Theoretical Yield = Actual Yield / (Percent Yield / 100)
Theoretical Yield = 50 / 0.80
Theoretical Yield = 62.5 g (the chemist must scale up the reaction to start with enough reactants to produce 62.5 g theoretically)
When to Use It
Use the percent yield formula whenever you need to evaluate or plan chemical reactions.
- Evaluating the success of a laboratory synthesis
- Comparing the efficiency of different reaction conditions or catalysts
- Planning how much starting material is needed to obtain a desired amount of product
- Industrial chemistry — optimizing reaction conditions for maximum yield
- Pharmaceutical manufacturing — yield affects production cost and feasibility
- Quality control — identifying process problems when yields drop below expected values