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Freezing point depression lab Essay

Using Freezing-Point Depression to Find Molecular Weight

In this lab, the purpose was to use the freezing point depression method to determine the molecular weight of aspirin. This was done by determining the freezing of t-butanol and that of a t-butanol and aspirin solution; then finding the molality of the solution, and moles of aspirin. In the results of the experiment, the 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. Introduction:

Freezing point depression defined as a vertical medium in reference to boiling point elevation. The general definition is the effect of lowering the freezing point of a substance due to an increased amount of solute added to the solvent. This effect is known as colligative property. The equation that shows this relationship is:

ΔT= Kf * mEq. 1
The goal of this experiment was to determine the molar mass of aspirin using the freezing point depression method. First, the freezing temperature of the pure solvent and t-butanol were determined. Then by adding a known mass of aspirin to a known mass of t-butanol, the lowering of the freezing temperature of the solution was determined. By measuring the freezing point depression, ΔT, and the mass of aspirin, the molar mass of the aspirin solute was determined.

Equations used in this experiment:
The different in freezing temperatures between t-butanol and mixture of t-butanol and aspirin:
ΔT= T1- T2Eq. 2
Where T1: temperature of t-butanol
T2: temperature of t-butanol and aspirin

The molar mass of aspirin:
Moles= mass/ molar massEq. 3
Table 1. Mass of all the measurements (g)

Trial 1Trial 2
Mass of test tube (g)
Mass of test tube+ t-butanol (g)
Mass of aspirin+ solvent+ test tube (g)20.504 20.670


Table 2. Freezing temperature of t-butanol and aspirin-butanol solution (°C)

Trial 1Trial 2
Freezing point of pure-butanol (°C)17.549.850
Freezing point of pure-butanol-aspirin solution (°C)

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: (748- 180.2)/180.2 * 100%= 315%

%Error for trial 2: (192.2- 180.2)/180.2 *100%= 6.7%


The addition of aspirin solute to the t-butanol solution caused the vapor pressure to decrease. This decrease in vapor pressure and increase in solute present caused the freezing point to became lower when mixed thin the solution. The purpose of this lab was to determine the molecular mass of aspirin by measuring the freezing point depression. This was done by reacting t-butanol with aspirin as a solute. A pure solvent of t-butanol’s freezing point was first determined by melting and freezing it and then the solute was added and the freezing point dropped. This difference was the freezing point depression of the solute. From this freezing point change, the molecular mass of the solute was calculated. The first trial with 0.200 g of the solute produced a molar mass of 748 g/mol and the second trial with 0.195 g of the solute produced a molar mass of 192.2 g/mol.

The molar mass of the first trial was way off from the accepted value, while the molar mass of the second trial was quite close to the accepted value. These results were confirmed when a percent error of 315% in for the first trial and 6.7% in the second trial. Errors that contributed to the percent error could include impurities in the aspirin and t-butanol mixture, mixing the two in the main beaker commonly used to heat the solutions, and the coexistence of solid and liquid forms of the solutions. Adding aspirin to t-butanol in the aspirin-butanol mixture, the kinetic energy of the solution was inhibited evident of a lower vapor pressure and decreasing freezing/melting point. One other error could be that not all of the aspirin was transferred from the weighting boat into the test tube or having the aspirin stuck on the walls of the test tube.

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