What is the theoretical yield of aspirin in your synthesis? The mole ratio is 1:1 between salicylic acid and aspirin in this reaction. The melting temperature of pure acetylsalicylic acid is 135°C. Based on the results of the melting temperature test, do you believe that your aspirin sample is pure? Explain. Based on the results of the absorbance testing with the spectrophotometer, what is the percent purity of your sample of aspirin? Does this percent purity compare well with the results of the melting temperature test? Explain. Use your percent purity calculations to determine the percent yield of your synthesis of aspirin.
The Correct Answer and Explanation is:
Theoretical Yield Calculation:
Let’s say you started with 2.00 g of salicylic acid (C₇H₆O₃). The molar mass of salicylic acid is approximately 138.12 g/mol.Moles of salicylic acid=2.00 g138.12 g/mol=0.01448 mol\text{Moles of salicylic acid} = \frac{2.00\ \text{g}}{138.12\ \text{g/mol}} = 0.01448\ \text{mol}Moles of salicylic acid=138.12 g/mol2.00 g=0.01448 mol
Given the 1:1 ratio, the theoretical moles of aspirin = 0.01448 mol.
The molar mass of aspirin (C₉H₈O₄) is approximately 180.16 g/mol:Theoretical yield of aspirin=0.01448 mol×180.16 g/mol=2.61 g\text{Theoretical yield of aspirin} = 0.01448\ \text{mol} \times 180.16\ \text{g/mol} = 2.61\ \text{g}Theoretical yield of aspirin=0.01448 mol×180.16 g/mol=2.61 g
Melting Point Test Interpretation:
Pure acetylsalicylic acid has a melting point of 135°C. If your sample melted at a lower or broader range, it indicates the presence of impurities. For example, if your sample melts between 128–132°C, the deviation suggests that the aspirin is not pure. A sharp, narrow melting range at 135°C would indicate a high degree of purity.
Percent Purity from Spectrophotometer:
Assume your spectrophotometer testing shows that your sample is 85% pure. This means that out of the total mass of recovered product, only 85% is pure aspirin. This aligns with the melting point result: both tests suggest some impurities are present.
Percent Yield Using Percent Purity:
Let’s say you isolated 2.40 g of crude aspirin. To find actual yield of pure aspirin, multiply by percent purity:Pure aspirin=2.40 g×0.85=2.04 g\text{Pure aspirin} = 2.40\ \text{g} \times 0.85 = 2.04\ \text{g}Pure aspirin=2.40 g×0.85=2.04 g
Then calculate percent yield:Percent yield=(2.04 g2.61 g)×100%=78.2%\text{Percent yield} = \left( \frac{2.04\ \text{g}}{2.61\ \text{g}} \right) \times 100\% = 78.2\%Percent yield=(2.61 g2.04 g)×100%=78.2%
Conclusion:
Your sample’s melting point and spectrophotometer results both indicate it is impure. The melting point is slightly lower than the pure compound, and the spectrophotometer confirms 85% purity. This moderate purity impacts the percent yield: while 2.40 g of product was collected, only 2.04 g was true aspirin, resulting in a 78.2% actual yield. These results are typical in student lab settings, where minor impurities or incomplete reactions are common.
