📘 IB Understanding
The percentage yield is calculated from the ratio of experimental yield to theoretical yield. It measures how efficiently a reaction was carried out in the laboratory.
The Formula
\( \text{Percentage Yield} = \frac{\text{Experimental Yield}}{\text{Theoretical Yield}} \times 100\% \)
Theoretical Yield
The maximum amount of product that could be formed from a given amount of the limiting reactant, assuming the reaction goes to completion with no losses. Calculated from stoichiometry.
Experimental Yield
The actual amount of product obtained and measured in the laboratory. Almost always less than the theoretical yield.
Worked Example
Step 1: Identify the values:
Experimental yield = 12.50 g
Theoretical yield = 14.35 g
Step 2: Apply the formula:
\( \text{% Yield} = \frac{12.50}{14.35} \times 100 = \) 87.1%
Why is the Yield Rarely 100%?
| Reason | Explanation |
|---|---|
| Incomplete reaction | The reaction may not go to completion, especially reversible reactions that reach equilibrium |
| Side reactions | Competing reactions produce unwanted by-products, reducing the amount of desired product |
| Transfer losses | Product is lost during transfer between containers, filtering, or washing |
| Impure reactants | If reactants contain impurities, less of the actual reactant is available |
⚠️ Can Yield Exceed 100%?
Experimentally, apparent yields above 100% can occur due to errors such as incomplete drying (residual water adds mass) or impurities in the product. This indicates experimental error, not a genuine excess of product.
🔗 Link to the IA
Percentage yield analysis is directly applicable to the Evaluation criterion of the IB Internal Assessment, where students must account for experimental uncertainties and explain deviations from theoretical predictions.