How To Calculate Percent Yield Of Aspirin

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

Synthesizing aspirin (acetylsalicylic acid) in a lab is a common organic chemistry experiment. Calculating the percent yield is crucial for assessing the efficiency of the synthesis. A high percent yield indicates a successful reaction, while a low yield suggests areas for improvement in the experimental procedure. This comprehensive guide will walk you through the process of calculating the percent yield of aspirin, explaining each step in detail.

Understanding Percent Yield

Percent yield represents the ratio of the actual yield (the amount of product obtained experimentally) to the theoretical yield (the maximum amount of product that could be obtained based on stoichiometry). It's expressed as a percentage and provides valuable insight into the success of a chemical reaction.

Formula:

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

Let's break down each component:

1. Actual Yield: The Reality of the Experiment

The actual yield is the mass (usually in grams) of pure aspirin you successfully isolated and collected at the end of your experiment. This is a measurable quantity, obtained after purification steps like recrystallization. It represents the amount of product you actually obtained in the lab. Important note: Any impurities present will reduce the actual yield because you're only considering the pure product.

2. Theoretical Yield: The Maximum Possible Outcome

The theoretical yield is the maximum amount of aspirin that could be produced based on the stoichiometry of the reaction and the limiting reagent. It represents the ideal scenario with 100% efficiency. To calculate this, you need:

Balanced Chemical Equation: The balanced chemical equation for aspirin synthesis is crucial. It shows the molar ratios of reactants and products. A typical synthesis uses salicylic acid and acetic anhydride:

C₇H₆O₃ (salicylic acid) + C₄H₆O₃ (acetic anhydride) → C₉H₈O₄ (aspirin) + CH₃COOH (acetic acid)
Moles of Limiting Reagent: Identify the limiting reagent. This is the reactant that is completely consumed first, thus limiting the amount of product that can be formed. You'll need the mass of the limiting reagent used and its molar mass to calculate the moles.
Molar Mass of Aspirin: You'll need the molar mass of aspirin (C₉H₈O₄) to convert moles of aspirin to grams.

Steps to calculate the theoretical yield:

Determine the moles of the limiting reagent: Moles = Mass (g) / Molar Mass (g/mol)
Use the stoichiometry from the balanced equation: The mole ratio between the limiting reagent and aspirin will dictate how many moles of aspirin can be produced. In our example, the ratio is 1:1.
Calculate the moles of aspirin produced: Moles of aspirin = Moles of limiting reagent x (Moles of aspirin / Moles of limiting reagent) (This will be 1 in our case)
Convert moles of aspirin to grams: Grams of aspirin = Moles of aspirin x Molar Mass of aspirin (g/mol)

This final gram amount is your theoretical yield.

A Step-by-Step Example: Calculating Percent Yield of Aspirin

Let's assume the following data from an aspirin synthesis experiment:

Mass of salicylic acid used: 2.00 g
Molar mass of salicylic acid: 138.12 g/mol
Mass of aspirin obtained (after purification): 2.50 g
Molar mass of aspirin: 180.16 g/mol

1. Calculate the moles of salicylic acid:

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

2. Determine the theoretical yield of aspirin:

Since the mole ratio of salicylic acid to aspirin is 1:1, the moles of aspirin produced will also be 0.0145 mol.

Grams of aspirin (theoretical yield) = 0.0145 mol x 180.16 g/mol = 2.61 g

3. Calculate the percent yield:

Percent Yield = (Actual Yield / Theoretical Yield) x 100% Percent Yield = (2.50 g / 2.61 g) x 100% = 95.8%

In this example, the percent yield of aspirin is 95.8%. This indicates a relatively efficient synthesis.

Factors Affecting Percent Yield

Several factors can influence the percent yield of a reaction, 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 yield of the desired product.
Loss of Product During Purification: Some product is inevitably lost during purification steps like filtration, recrystallization, and drying.
Impurities in Reactants: Impurities in the starting materials can interfere with the reaction and lower the yield.
Experimental Error: Errors in measurement, technique, or equipment can affect the yield.

Improving Percent Yield

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

Purity of Reagents: Ensure that the starting materials (salicylic acid and acetic anhydride) are pure.
Reaction Conditions: Optimize reaction conditions like temperature and reaction time.
Efficient Purification: Use appropriate techniques to maximize product recovery during purification.
Minimize Losses: Handle the product carefully to reduce losses during transfers and handling.
Careful Measurement: Precisely measure the reactants to ensure accurate stoichiometry.

Advanced Considerations: Dealing with Limiting Reagents and Excess Reagents

In many reactions, one reactant is used in excess to ensure that the other reactant (the limiting reagent) is completely consumed. This often improves the yield by driving the reaction towards completion. However, calculating the theoretical yield still relies on the amount of the limiting reagent. The excess reagent does not factor into the theoretical yield calculation.

Conclusion

Calculating the percent yield of aspirin provides a quantitative measure of the efficiency of your synthesis. By understanding the factors affecting yield and implementing strategies for improvement, you can refine your experimental technique and achieve higher yields in future syntheses. Remember that a high percent yield isn’t just about the numbers; it demonstrates a mastery of experimental techniques and a deep understanding of the chemical reaction. Through careful planning, precise execution, and meticulous attention to detail, you can consistently improve the efficiency of your aspirin synthesis and achieve impressive yields.