Percent Yield & Percent Error

Related Examples and Practice Problems

Additional Worked Out Examples/ Practice

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  • Separation techniques: Selected and explaining limitation of appropriate separation

  • Relating Properties to Composition: Predicting classification based on descriptive properties

Topic Summary & Highlights
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Core Concept

Percent yield and percent error are two metrics commonly used to evaluate the success of chemical experiments. Percent yield measures the efficiency of a reaction, while percent error assesses the accuracy of experimental results compared to theoretical values.

$\text{Percent Error} = \left( \frac{|\text{Experimental Value} - \text{Theoretical Value}|}{\text{Theoretical Value}} \right) \times 100\%$

$\text{Percent Yield} = \left( \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \right) \times 100\%$

Practice Tips

  • Always Start with a Balanced Equation: For percent yield, stoichiometric calculations require a balanced equation to accurately determine the theoretical yield.

  • Use Consistent Units: Ensure actual yield and theoretical yield are in the same units for percent yield calculations.

  • Interpret Percent Yield:

    • A yield over 100% suggests experimental error, such as contamination.

    • A low percent yield can result from side reactions, incomplete reactions, or losses during product recovery.

  • Interpret Percent Error:

    • A lower percent error indicates greater accuracy.

    • High percent error suggests significant deviation from the accepted or theoretical value, potentially due to measurement errors or procedural flaws.

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Steps to Calculate Percent Yield

  1. Determine the Theoretical Yield:

    • Use stoichiometry based on the balanced chemical equation to calculate the theoretical yield of the product.

  2. Measure the Actual Yield:

    • Measure the mass or volume of the product obtained from the experiment.

  3. Calculate Percent Yield:

    • Plug the actual yield and theoretical yield into the percent yield formula.

Steps to Calculate Percent Error

  1. Determine the Theoretical (Accepted) Value:

    • Use stoichiometric calculations or accepted reference values for the expected result.

  2. Measure the Experimental Value:

    • Record the value obtained from the experiment.

  3. Calculate Percent Error:

    • Substitute the experimental value and theoretical value into the percent error formula.

Key Terms

  1. Theoretical Yield:

    • The maximum amount of product that can be formed in a reaction, based on stoichiometric calculations.

    • Calculated assuming complete reaction with no loss of materials.

  2. Actual Yield:

    • The amount of product actually obtained from a reaction.

    • Often less than the theoretical yield due to practical limitations, side reactions, or loss of product.

  3. Percent Yield:

    • A measure of the efficiency of a reaction, showing how close the actual yield is to the theoretical yield.

    • Calculated using the formula: $\text{Percent Yield} = \left( \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \right) \times 100\%$

    • A percent yield close to 100% indicates an efficient reaction.

  4. Percent Error:

    • A measure of the accuracy of an experimental value compared to a theoretical or accepted value.

    • Calculated using the formula: $\text{Percent Error} = \left( \frac{|\text{Experimental Value} - \text{Theoretical Value}|}{\text{Theoretical Value}} \right) \times 100\%$

    • A smaller percent error indicates higher accuracy.

Example Problem: Percent Yield

Problem: In a reaction, the theoretical yield of NaCl is calculated to be 15.0 g. After the reaction, only 12.5 g of NaCl is actually obtained. Calculate the percent yield.

Solution:

  1. Identify Theoretical and Actual Yields:

    • Theoretical Yield = 15.0 g

    • Actual Yield = 12.5 g

  2. Use the Percent Yield Formula:

    • Percent Yield = (Actual Yield / Theoretical Yield) * 100%

    • Percent Yield = (12.5 g / 15.0 g) * 100% = 83.3%

Answer: The percent yield is 83.3%.

Example Problem: Percent Error

Problem: A student measures the density of a metal as 7.85 g/cm³. The accepted density for this metal is 7.92 g/cm³. Calculate the percent error.

Solution:

  1. Identify Experimental and Theoretical Values:

    • Experimental Value = 7.85 g/cm³

    • Theoretical Value = 7.92 g/cm³

  2. Use the Percent Error Formula:

    • Percent Error = (|Experimental Value - Theoretical Value| / Theoretical Value) * 100%

    • Percent Error = (|7.85 g/cm³ - 7.92 g/cm³| / 7.92 g/cm³) * 100% = (0.07 g/cm³ / 7.92 g/cm³) * 100% = 0.88%

Answer: The percent error is 0.88%.

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