The percent yield is calculated by dividing the actual yield (the amount of product you actually obtained) by the theoretical yield (the amount of product you should have obtained based on stoichiometry) and then multiplying by 100%.
Here's a breakdown of the process:
Understanding the Key Terms
- Actual Yield: The amount of product you actually obtain from a chemical reaction. This is a measured value and is usually less than the theoretical yield due to various factors like incomplete reactions, side reactions, and loss of product during purification.
- Theoretical Yield: The amount of product you should obtain from a chemical reaction based on the stoichiometry of the balanced chemical equation, assuming perfect reaction conditions and no loss of product. This is a calculated value.
- Percent Yield: The efficiency of a chemical reaction, expressed as a percentage. It indicates how much of the expected product was actually obtained.
The Formula
The formula for calculating percent yield is:
Percent Yield = (Actual Yield / Theoretical Yield) * 100%Steps to Calculate Percent Yield
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Write the Balanced Chemical Equation: Make sure you have a correctly balanced chemical equation for the reaction. This is essential for determining the stoichiometric relationships between reactants and products.
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Determine the Theoretical Yield:
- Start with the amount of the limiting reactant (the reactant that runs out first).
- Use stoichiometry (mole ratios from the balanced equation) to calculate the theoretical yield of the desired product in moles.
- Convert the theoretical yield from moles to grams (or other appropriate units) using the molar mass of the product.
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Determine the Actual Yield: The actual yield will be given in the problem or determined experimentally. Make sure the actual yield is in the same units as the theoretical yield (e.g., grams).
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Calculate the Percent Yield: Plug the actual yield and theoretical yield into the formula:
Percent Yield = (Actual Yield / Theoretical Yield) * 100%
Example
Let's say you perform a reaction where you should theoretically produce 25 grams of a product (theoretical yield). However, after performing the reaction and isolating the product, you only obtain 20 grams (actual yield).
To calculate the percent yield:
Percent Yield = (20 grams / 25 grams) * 100% = 80%This means the reaction had an 80% yield.
Factors Affecting Percent Yield
Several factors can cause the actual yield to be less than the theoretical yield:
- Incomplete Reactions: Not all reactants may react to form products.
- Side Reactions: Reactants may participate in other, unintended reactions.
- Loss During Purification: Some product may be lost during separation and purification steps (e.g., filtration, distillation).
- Experimental Error: Measurement errors can also affect the accuracy of the actual yield.
Significance of Percent Yield
The percent yield is an important indicator of the effectiveness of a chemical reaction. A high percent yield indicates that the reaction is efficient and that minimal product is lost. A low percent yield indicates that the reaction is inefficient and that significant product is being lost or that unwanted side reactions are occurring.
