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The purpose of this lab was to use the freezing point depression method to determine the molecular weight of aspirin. This was achieved by measuring the freezing point of t-butanol and a t-butanol and aspirin solution, calculating the molality of the solution, and determining the moles of aspirin. The experimental molar mass was found to be 192.2 g/mol, which differed from the established value of 180.2 g/mol by 6.7% error.
Freezing point depression is a colligative property that refers to the lowering of the freezing point of a solvent due to the addition of a solute.
This property can be quantified using the equation:
ΔT = Kf * m (Equation 1)
Where ΔT is the freezing point depression, Kf is the cryoscopic constant, and m is the molality of the solution. In this experiment, we aimed to determine the molar mass of aspirin by measuring the freezing point depression of a t-butanol and aspirin solution. This involved measuring the freezing points of pure t-butanol and the t-butanol-aspirin solution and using the difference to calculate the molar mass of aspirin.
The equations used in this experiment include:
The difference in freezing temperatures between t-butanol and the mixture:
ΔT = T1 - T2 (Equation 2)
Where T1 is the temperature of pure t-butanol, and T2 is the temperature of the t-butanol and aspirin mixture.
The moles of aspirin:
Moles = mass / molar mass (Equation 3)
Materials:
Methods:
Table 1. Mass Measurements (g)
| Trial 1 | Trial 2 | |
|---|---|---|
| Mass of test tube (g) | 18.32 | 18.32 |
| Mass of test tube + t-butanol (g) | 20.54 | 20.41 |
| Mass of aspirin + solvent + test tube (g) | 20.50 | 20.67 |
Table 2. Freezing Temperature Measurements (°C)
| Trial 1 | Trial 2 | |
|---|---|---|
| Freezing point of pure t-butanol (°C) | 17.54 | 9.85 |
| Freezing point of t-butanol-aspirin solution (°C) | 8.78 | 5.82 |
The experimental molar mass of aspirin for Trial 1: 748 g/mol
The experimental molar mass of aspirin for Trial 2: 192.2 g/mol
% Error for Trial 1: 315%
% Error for Trial 2: 6.7%
The addition of aspirin to the t-butanol solution caused a decrease in vapor pressure and, consequently, a lower freezing point.
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This is a characteristic behavior of solutions and is in accordance with the colligative property of freezing point depression.
The goal of this experiment was to determine the molecular mass of aspirin by measuring the freezing point depression of t-butanol caused by the addition of aspirin. In Trial 1, where 0.200 g of aspirin was used, the calculated molar mass was 748 g/mol. In Trial 2, with 0.195 g of aspirin, the calculated molar mass was 192.2 g/mol. These values show a significant discrepancy.
The high percent error in Trial 1 (315%) suggests that there were significant errors or inconsistencies in the experimental procedure. Possible sources of error include impurities in the aspirin and t-butanol mixture, incomplete mixing of the solute and solvent, and the coexistence of solid and liquid phases in the solution.
In contrast, Trial 2 yielded a molar mass close to the accepted value (180.2 g/mol), with a percent error of 6.7%. This suggests that Trial 2 was conducted more accurately or that the amount of aspirin used was closer to the ideal amount.
It's essential to note that colligative properties like freezing point depression are sensitive to the concentration of solute and can be affected by small variations in experimental conditions. Additionally, the accuracy of measurements and the purity of reagents play a crucial role in obtaining reliable results.
In this experiment, we aimed to determine the molecular weight of aspirin using the freezing point depression method. Two trials were conducted, and the results differed significantly. Trial 1 produced a molar mass of 748 g/mol with a high percent error of 315%, while Trial 2 yielded a molar mass of 192.2 g/mol with a lower percent error of 6.7%. These discrepancies suggest that errors occurred in the experimental procedure, including the handling of impurities and the amount of aspirin used.
To improve the accuracy of future experiments, it is essential to ensure the purity of reagents, thoroughly mix the solute and solvent, and carefully control the experimental conditions. Conducting multiple trials and averaging the results can also help reduce errors and improve the reliability of the calculated molar mass.
Using Freezing-Point Depression to Find Molecular Weigh. (2016, Apr 19). Retrieved from https://studymoose.com/document/freezing-point-depression-lab
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