The Freezing Point Of P Xylene Essay

The Freezing Point Of P Xylene Essay

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The freezing point of p-xylene was calculated as 13.29C after averaging the data that appeared on Graph 1 once the temperature leveled off. With this value, the Tf for each trial was able to be calculated through Equation 1, which led to Kf being calculated in Equation 2. Both equations were able to be used given that the measurements were in terms of molality, which is not temperature dependent. After completing calculations, the average Kf of the three trials of the p-xylene and toluene solution was computed as as 4.56(C/m) as shown in Table 1, however, the theoretical value was slightly lower than calculated, 4.3(C/m). This resulted in a 6.04% error as shown in Equation 5. Possible causes of error could have resulted from adding too much solute or too little solvent; the calculations then would have been off given that the solution was not diluted enough since freezing point depression is a colligative property. Having too much solvent could be attributed to having a higher Kf than the theoretical value, given that the solute would not have as much impact on the pure substance, p-xylene. This would raise the Kf value, bringing it closer to the solvent’s Kf, given that Kf is a constant that is solvent specific. Another source of error resulting in the higher Kf could have resulted from contamination of the solvent or solute, given that it could introduce a polar substance or different molecules which would alter the hydrogen bonding.
Using the previously calculated Kf, the molar masses of unknown substances A, C, and D were able to be calculated. However, given that the original Kf was slightly larger than the theoretical value, the molar masses were significantly higher than the masses using the theoretical value as shown...


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...s measured as well as the mass of 10 drops of toluene, the solute. The temperature of the solution rose to the freezing point after supercooling, then continued to drop as the solution froze, versus stabilizing as the pure p-xylene did. The maximum temperature obtained after supercooling was recorded as the freezing point of the solution. This process was repeated three times, each with a new test tube and the same beaker and scale for measuring the masses, then the Tf was calculated for each trial, as well as the average Kf. For the final portion of the lab, the same procedure as above was followed with the substation of 10 drops of unknown solutes instead of toluene in the p-xylene. Each unknown, A, C, and D, had one trial each. With the experimentally gathered data, the molar masses were then computed and compared to the given compound molar masses to identify.

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