Experimental Yield vs Theoretical Yield
The percentage yield is calculated from the ratio of actual experimental yield to theoretical maximum yield. It serves as a benchmark for experimental competence and industrial process efficiency.
The Formula
The Percentage Yield Formula
\( \text{Percentage Yield} = \dfrac{\text{Experimental Yield}}{\text{Theoretical Yield}} \times 100\% \)
Theoretical Yield
The maximum calculated amount of product that could form if 100% of the limiting reagent reacted completely with zero side reactions and zero mechanical loss.
Experimental (Actual) Yield
The actual quantity of purified product obtained and weighed in the laboratory following experimental synthesis, filtration, and drying.
Worked Example
Calculating Percentage Yield for Silver Chloride Precipitation
Problem: A student reacts \(5.85\text{ g}\) of \(\text{NaCl}\) with excess \(\text{AgNO}_3\). The theoretical yield of \(\text{AgCl}\) is \(14.35\text{ g}\), but only \(12.50\text{ g}\) is obtained. Calculate the percentage yield.
Calculation:
\(\text{Percentage Yield} = \dfrac{12.50\text{ g}}{14.35\text{ g}} \times 100 = \mathbf{87.1\%}\)
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%? (Experimental Artifacts)
Experimentally, apparent yields above 100% can occur due to experimental errors: incomplete drying leaving residual solvent/water, or unseparated contaminants/by-products adding spurious mass. True chemical yield cannot exceed 100%.
Applying Percentage Yield in IA Evaluations
Percentage yield analysis is directly applicable to the Evaluation criterion of your IB Chemistry Internal Assessment. Discussing specific sources of yield loss (incomplete reaction, mechanical transfer loss, equilibrium limitations) demonstrates rigorous analytical evaluation.
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