How To Calculate Percentage Yield Of Aspirin

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

Synthesizing aspirin in a laboratory setting is a common organic chemistry experiment. However, achieving a perfectly theoretical yield is rarely the case. Understanding how to calculate the percentage yield of aspirin is crucial for evaluating the efficiency of your synthesis and identifying areas for improvement. This comprehensive guide will walk you through the process, explaining each step clearly and concisely.

Understanding Percentage Yield

Before delving into the calculations, let's clarify the concept of percentage yield. It represents the ratio of the actual yield (the amount of product you actually obtained) to the theoretical yield (the maximum amount of product you could have obtained based on stoichiometry) expressed as a percentage. A higher percentage yield indicates a more efficient reaction.

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

This formula is the cornerstone of our calculations. Let's break down how to determine each component.

Determining the Theoretical Yield of Aspirin

The theoretical yield is the maximum amount of aspirin you could produce given the starting materials and the stoichiometry of the reaction. Aspirin synthesis involves the esterification of salicylic acid with acetic anhydride. The balanced chemical equation is:

C₇H₆O₃ (salicylic acid) + C₄H₆O₃ (acetic anhydride) → C₉H₈O₄ (aspirin) + CH₃COOH (acetic acid)

From this equation, we can see a 1:1 molar ratio between salicylic acid and aspirin. This means that one mole of salicylic acid will, in theory, produce one mole of aspirin. However, we rarely work with moles directly in the lab. Instead, we use grams. Therefore, we need to convert grams of salicylic acid to moles and then use the molar ratio to calculate the theoretical yield of aspirin in grams.

Step-by-Step Calculation of Theoretical Yield:

1. Find the molar mass of salicylic acid (C₇H₆O₃): Add the atomic masses of all atoms in the molecule. (12.01 x 7) + (1.01 x 6) + (16.00 x 3) = 138.12 g/mol

2. Calculate the moles of salicylic acid: Divide the mass of salicylic acid used (in grams) by its molar mass (138.12 g/mol). For example, if you used 2.00 grams of salicylic acid:

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

3. Determine the moles of aspirin (C₉H₈O₄): Based on the stoichiometry, the moles of aspirin produced will be equal to the moles of salicylic acid used (assuming a 1:1 ratio and 100% reaction efficiency). In our example:

   Moles of aspirin = 0.0145 moles

4. Calculate the molar mass of aspirin (C₉H₈O₄): (12.01 x 9) + (1.01 x 8) + (16.00 x 4) = 180.16 g/mol

5. Calculate the theoretical yield of aspirin in grams: Multiply the moles of aspirin by its molar mass:

   Theoretical yield = 0.0145 moles x 180.16 g/mol = 2.61 grams

Therefore, if you started with 2.00 grams of salicylic acid, the theoretical yield of aspirin would be 2.61 grams. Remember to adjust these calculations based on the actual mass of salicylic acid you used in your experiment.

Determining the Actual Yield of Aspirin

The actual yield is simply the mass of pure, dry aspirin you obtained after the synthesis, purification (recrystallization is commonly used), and drying process. This is determined by weighing your purified aspirin product. Accuracy in this step is critical for an accurate percentage yield calculation.

Important Considerations for Accurate Actual Yield Measurement:

• Complete Drying: Ensure your aspirin crystals are completely dry before weighing. Residual solvent can significantly affect the mass.
• Pure Product: Impurities will inflate the measured mass, leading to an artificially high percentage yield. Recrystallization is vital to ensure a pure product.
• Accurate Weighing: Use an analytical balance for precise mass measurements.

Calculating the Percentage Yield

Now that we have both the actual yield (obtained from weighing your purified aspirin) and the theoretical yield (calculated previously), we can finally calculate the percentage yield using the formula:

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

Let's say you obtained 1.80 grams of pure, dry aspirin after recrystallization. Using our previous example:

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

This means your aspirin synthesis had a 69% yield. This value indicates the efficiency of your reaction; several factors can influence this.

Factors Affecting Percentage Yield

Several factors can contribute to a lower-than-expected percentage yield in aspirin synthesis. Understanding these factors can help you optimize your experimental procedure for higher yields in future experiments.

1. Incomplete Reaction:

• Reaction Time: Insufficient reaction time may prevent the complete conversion of salicylic acid to aspirin.
• Temperature: The reaction is temperature-sensitive. Too low a temperature can slow down the reaction, whereas too high a temperature could cause side reactions or decomposition.
• Reactant Ratio: Using an insufficient amount of acetic anhydride can limit the reaction.

2. Loss of Product During Purification:

• Recrystallization Losses: Some product is inevitably lost during the recrystallization process, due to solubility losses in the solvent.
• Filtration Losses: Some product can adhere to the filter paper during filtration.
• Transfer Losses: Small amounts of product can be lost during transfers between containers.

3. Presence of Impurities:

• Unreacted Starting Materials: Incomplete reaction leads to the presence of unreacted salicylic acid.
• Byproducts: Side reactions can produce unwanted byproducts that contaminate the aspirin.

4. Experimental Errors:

• Inaccurate Measurements: Inaccurate weighing of reactants or product can significantly affect the yield calculation.
• Improper Techniques: Inconsistent stirring or heating can affect the reaction efficiency.

Improving Percentage Yield

By carefully controlling experimental parameters and using appropriate techniques, you can significantly improve the percentage yield of your aspirin synthesis:

• Optimize Reaction Conditions: Ensure that the reaction is carried out at the optimal temperature and for a sufficient amount of time.
• Use Excess Acetic Anhydride: Using an excess of acetic anhydride can drive the reaction towards completion.
• Improve Purification Techniques: Careful recrystallization techniques can minimize product loss. Consider using a different recrystallization solvent for higher recovery.
• Accurate Measurements: Ensure all measurements are accurate and precise using calibrated equipment.
• Efficient Handling: Minimize transfer losses by using appropriate techniques and equipment.

Conclusion

Calculating the percentage yield is an essential aspect of any chemical synthesis. Understanding the factors that influence yield helps in refining experimental procedures and achieving higher yields. By following the steps outlined above and paying close attention to detail throughout the synthesis and purification process, you can obtain a more accurate and reliable percentage yield of aspirin. Remember that consistent practice and attention to detail are key to achieving high yields in organic chemistry experiments. This comprehensive guide provides a solid foundation for understanding and improving the efficiency of your aspirin synthesis.