[1]

Freezing Point Depression: Lauric Acid

Introduction

When a substance freezes or solidifies, the molecules form a certain arrangement which in most cases is a closely packed and very dense solid. However when impurities are introduced, they interfere with normal molecular interactions. This causes the need for more energy to be taken out of the system in order for the system to solidify. Hence, the freezing temperature is lowered. An example of this process can be seen during winter after a snowstorm. Road salt is spread onto roads in order to lower the freezing point of water so that ice cannot form as easily.

In this lab, Lab Pro was used to determine the freezing point depression constant (K f ) for Lauric acid. This was done by first determining the freezing point of pure Lauric acid. In order to do so, we obtained a test tube with pure Lauric acid which was initially frozen. We melted the solid by placing it in a beaker of water at around 60℃. Then, the test tube was placed in a beaker of cold water and a temperature probe was placed in the test tube. As the Lauric acid cooled and eventually formed a solid, the temperature data was recorded by the Lab Pro device. Once the Lauric acid was completely frozen into a solid, the graph and data were able to be analyzed. This process was repeated twice to find the average freezing point of Lauric Acid.

The first part of the experiment was done only to find the freezing point of pure Lauric acid, which will serve as a baseline for the following two experiments and eventually lead to the determination of K f for Lauric acid. The following two experiments were done exactly as the first part. The sample was frozen to begin with, melted in warm water, and put in an ice bath to solidify while temperature data was recorded. However, the samples in Part 2 were not pure Lauric acid, but a solution of Lauric acid and Benzoic Acid. In the first test tube, the mixture is composed of 0.750g of Benzoic acid and 8.00g of Lauric acid. In the second tube, the mixture was 1.50g BA and 8.00g LA. The molality and change in freezing temperatures can be easily calculated and thus, the freezing point depression constant can be found.

Equations

The most important equation in this lab is as follows:

∆T=K f∗m

Where ∆T is the lowering of the freezing point in ℃ for the solution relative to the freezing point of the pure solid, m is the molality of the solution, and K f is the freezing point depression constant for the solvent. This equation was crucial and utilized in Part 2 of this lab to find K f by first finding the ∆T that resulted from the two impure mixtures.

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As shown in the previous equation, m represents the molality of the solution and can be calculated by the following equation:

molality=moles solutemass solvent

The molality is used rather than the molarity because molality does not depend on the volume of the solvent which can change when temperature changes.

Results

The freezing point of pure Lauric Acid was found in the first part of this lab by analyzing the graph of temperature vs. time. One trial is shown to the right in Figure 1. The freezing point can be seen as the flat portion of the graph. The process was done two times and the two freezing points were found to be 43.44℃ and 43.13℃, which leads to an average freezing point of 43.285℃ for pure Lauric acid.

In the next two experiments, the molality of the mixtures were found by using the molar weight of Benzoic Acid which is 122.12 g/mol. The molality for the 0.750g BA sample was found to be 0.768mol/kg. Also, the freezing point for this sample was found to be 40.99℃, which corresponds to a ∆T=2.295℃. Furthermore, using equation [1], the freezing point depression constant was found to be K f=2.989℃/m .

Similarly, the molality for 1.50g BA sample was found to be 1.534mol/kg. Also, the freezing point was found to be 37.85℃ which corresponds to ∆T=¿ 5.435℃. This leads to the calculation of K f=3.540℃/m for the second sample. The average was determined to be K f=3.265℃/m, which was the result of interest.

Discussion

The results of this lab were qualitatively expected. After the freezing point was determined for pure Lauric Acid, lower freezing points were found each time for the two samples with BA solutes. This was expected because the impurities in the solution cause the need for more energy to be taken out of the system for the system to solidify. As for the quantitative results, the freezing point of pure Lauric Acid was found to be 43.285℃ which is pretty much exactly the same as the literature value at 43.2℃.

Figure 1: Temperature vs. Time for Pure Lauric Acid

Considering the freezing point that we found was almost identical to the literature value, it is safe to assume that there was little to almost no error involved with our experiments. Since each part of the experiment was done exactly the same, we can assume that our value of K f=3.265℃/m is reasonably close to the actual freezing point depression constant of Lauric Acid.

Conclusion

With the use of Lab Pro, we were able to measure the decreasing temperatures of our 4 samples as they solidified. First, we found the average freezing point of pure Lauric Acid to be 43.285℃ which was almost identical to the literature value. Then, we calculated the freezing points of two solutions of Benzoic Acid and Lauric Acid and found that they were lower with the additional solute. This led to the determination of the freezing point depression constant of K f=3.265℃/mfor Lauric acid, which was the objective of the experiment.