How to Calculate Percent Yield of Aspirin

In chemistry, especially when synthesizing compounds like aspirin, understanding the efficiency of your reaction is crucial. This is where percent yield comes into play. It's a measure of how successful a chemical reaction was in producing the desired product compared to the maximum amount theoretically possible.

What is Percent Yield?

Percent yield is a fundamental concept in stoichiometry that helps chemists evaluate the effectiveness of a synthetic procedure. It compares the actual amount of product obtained from an experiment (actual yield) to the maximum possible amount that could be produced based on the balanced chemical equation (theoretical yield).

• Actual Yield: The mass of the product obtained from a chemical reaction in the laboratory. This is an experimentally determined value.
• Theoretical Yield: The maximum mass of product that could be formed from the given amounts of reactants, assuming 100% efficiency and no loss. This value is calculated using stoichiometry.

The Percent Yield Formula

The formula for calculating percent yield is straightforward:

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

Both the actual and theoretical yields must be in the same units (typically grams).

Step-by-Step Calculation for Aspirin

Let's walk through an example using the synthesis of aspirin (acetylsalicylic acid) from salicylic acid and acetic anhydride.

The Reaction:

Salicylic Acid + Acetic Anhydride → Acetylsalicylic Acid (Aspirin) + Acetic Acid

C7H6O3 + C4H6O3 → C9H8O4 + C2H4O2

Molar masses:

• Salicylic Acid (C₇H₆O₃): 138.12 g/mol
• Acetic Anhydride (C₄H₆O₃): 102.09 g/mol
• Aspirin (C₉H₈O₄): 180.16 g/mol

Example Scenario:

Suppose you start with 2.00 grams of salicylic acid and an excess of acetic anhydride, and after the reaction and purification, you isolate 1.50 grams of aspirin.

Step 1: Determine the Theoretical Yield

First, you need to calculate the theoretical yield of aspirin. Since acetic anhydride is in excess, salicylic acid is the limiting reactant.

1. Convert grams of limiting reactant to moles:
   Moles of Salicylic Acid = 2.00 g / 138.12 g/mol = 0.01448 mol
2. Use mole ratio from balanced equation to find moles of product:
   From the balanced equation, 1 mole of salicylic acid produces 1 mole of aspirin.
   Moles of Aspirin = 0.01448 mol Salicylic Acid × (1 mol Aspirin / 1 mol Salicylic Acid) = 0.01448 mol Aspirin
3. Convert moles of product to grams (Theoretical Yield):
   Theoretical Yield of Aspirin = 0.01448 mol × 180.16 g/mol = 2.609 g

So, the theoretical yield of aspirin for this experiment is 2.609 grams.

Step 2: Identify the Actual Yield

The actual yield is the amount of product you collected experimentally. In our example, you isolated 1.50 grams of aspirin.

Step 3: Apply the Percent Yield Formula

Now, plug the actual and theoretical yields into the formula:

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

Percent Yield = (1.50 g / 2.609 g) × 100%

Percent Yield = 0.5749 × 100%

Percent Yield = 57.49%

The percent yield for this aspirin synthesis is approximately 57.5%.

Why is Percent Yield Important?

A chemist's goal is often to achieve the highest possible percent yield. A low percent yield can indicate several issues:

• Incomplete Reaction: Not all reactants converted to products.
• Side Reactions: Reactants forming undesired byproducts instead of the main product.
• Loss During Purification: Product lost during filtration, washing, drying, or recrystallization steps.
• Experimental Errors: Inaccurate measurements, spills, or improper technique.
• Equilibrium Limitations: Some reactions are reversible and do not go to completion.

Analyzing the percent yield helps chemists optimize reaction conditions, improve purification methods, and identify potential problems in their experimental procedure.

Improving Percent Yield

To maximize percent yield, chemists often consider:

• Optimizing reaction temperature and time.
• Using appropriate catalysts.
• Careful handling and purification techniques to minimize product loss.
• Ensuring accurate measurement of reactants.
• Choosing the correct solvent system.

Conclusion

Calculating the percent yield of aspirin, or any chemical product, is an essential skill in chemistry. It provides a quantitative measure of your reaction's efficiency and offers insights into potential areas for improvement. By understanding and applying the percent yield formula, you can better evaluate your experimental results and refine your synthetic processes.