How To Calculate Percent Yield Of Aspirin

How to Calculate the Percent Yield of Aspirin: A Comprehensive Guide

Aspirin, or acetylsalicylic acid, is a common analgesic and antipyretic drug synthesized through the esterification of salicylic acid with acetic anhydride. Calculating the percent yield of aspirin produced during a synthesis experiment is crucial in evaluating the efficiency of the reaction. This comprehensive guide will walk you through the process, explaining each step clearly and providing helpful tips for accurate results.

Understanding Percent Yield

Before diving into the calculation, let's define what percent yield represents. Percent yield is the ratio of the actual yield (the amount of product obtained in the experiment) to the theoretical yield (the maximum amount of product that could be obtained based on stoichiometry) expressed as a percentage. It's a critical indicator of the success of a chemical reaction. A high percent yield signifies a highly efficient reaction, while a low percent yield suggests potential issues with the experimental procedure or side reactions.

The formula for calculating percent yield is:

Percent Yield = (Actual Yield / Theoretical Yield) x 100%

Determining the Actual Yield

The actual yield is the mass of pure, dry aspirin you successfully isolate and collect at the end of your experiment. This requires careful purification steps, such as recrystallization, to remove impurities and ensure an accurate measurement.

Steps to determine actual yield:

1. Purification: After the synthesis, your aspirin will likely be impure. Recrystallization is a common technique to purify aspirin. This involves dissolving the crude aspirin in a hot solvent (often ethanol or a mixture of ethanol and water), allowing it to cool slowly, and filtering out the crystallized aspirin. This process helps remove impurities that remain in solution.

2. Drying: After recrystallization, the purified aspirin crystals must be thoroughly dried. This is typically done by allowing the crystals to air dry or by using a vacuum desiccator. Ensure all the solvent is removed before weighing.

3. Weighing: Once completely dry, carefully weigh the purified aspirin crystals using an analytical balance. Record this weight in grams. This is your actual yield.

Calculating the Theoretical Yield

The theoretical yield is the maximum amount of aspirin that could be produced based on the starting materials and the balanced chemical equation. This calculation relies on stoichiometry, which involves using mole ratios from the balanced equation.

The balanced chemical equation for the synthesis of aspirin is:

C₇H₆O₃ (salicylic acid) + (CH₃CO)₂O (acetic anhydride) → C₉H₈O₄ (aspirin) + CH₃COOH (acetic acid)

Steps to determine theoretical yield:

1. Determine the limiting reactant: Identify the limiting reactant. The limiting reactant is the reactant that is completely consumed first, thus limiting the amount of product that can be formed. This often requires comparing the moles of each reactant present. You'll need the mass of the salicylic acid used and its molar mass (138.12 g/mol) to calculate the moles of salicylic acid. Similarly, calculate the moles of acetic anhydride used, using its molar mass (102.09 g/mol).

2. Calculate the moles of aspirin: Based on the balanced equation, one mole of salicylic acid reacts to produce one mole of aspirin. Use the moles of the limiting reactant (calculated in step 1) to determine the theoretical moles of aspirin that can be formed.

3. Convert moles of aspirin to grams: Use the molar mass of aspirin (180.16 g/mol) to convert the moles of aspirin calculated in step 2 into grams. This is your theoretical yield.

Example Calculation:

Let's assume you started with 2.00 g of salicylic acid and an excess of acetic anhydride.

1. Moles of salicylic acid: 2.00 g / 138.12 g/mol = 0.0145 moles

2. Moles of aspirin (theoretical): Since the mole ratio is 1:1, the theoretical moles of aspirin are also 0.0145 moles.

3. Theoretical yield of aspirin: 0.0145 moles x 180.16 g/mol = 2.61 g

In this example, the theoretical yield of aspirin is 2.61 g.

Calculating the Percent Yield: Putting It All Together

Now that you have both the actual yield (from your experiment) and the theoretical yield (from the calculations), you can finally calculate the percent yield.

Let's say your actual yield of purified aspirin was 1.80 g. Using the theoretical yield calculated above (2.61 g), the percent yield would be:

Percent Yield = (1.80 g / 2.61 g) x 100% = 69%

This means that your experiment yielded 69% of the maximum possible amount of aspirin.

Factors Affecting Percent Yield

Several factors can influence the percent yield of an experiment, including:

• Incomplete Reaction: The reaction may not go to completion, leaving some reactants unreacted.
• Side Reactions: Unwanted side reactions can consume reactants and produce byproducts, reducing the amount of desired product.
• Loss of Product During Purification: Some product might be lost during the purification steps, such as recrystallization or filtration.
• Impurities in Reactants: Impurities in starting materials can interfere with the reaction and decrease the yield.
• Experimental Error: Errors in measurement, temperature control, or reaction time can all affect the yield.

Improving Percent Yield

To improve the percent yield of your aspirin synthesis, consider these factors:

• Optimize reaction conditions: Ensure appropriate reaction temperature and time.
• Use pure reactants: Employ high-quality reagents to minimize impurities.
• Careful purification techniques: Use effective purification methods to minimize product loss.
• Efficient experimental technique: Minimize errors in measurement and handling.

Conclusion

Calculating the percent yield of aspirin is a vital part of understanding the efficiency of the synthesis process. By carefully determining the actual and theoretical yields and applying the formula, you can gain valuable insights into the effectiveness of your experiment and identify areas for improvement. Remember to meticulously record all data and to account for potential sources of error. A well-executed experiment with a high percent yield is a testament to careful planning and accurate execution. Mastering this calculation will significantly enhance your understanding of stoichiometry and practical laboratory techniques.