Abstract: The Purpose Of This Lab Was To Use The Freezing Po

Abstractthe Purpose Of This Lab Was To Use The Freezing Point Depress

Abstract: The purpose of this lab was to use the freezing point depression method to determine the freezing temperature of pure cyclohexane and the freezing temperature of a solution of biphenyl and cyclohexane. Additionally, the experiment aimed to use the relationship between freezing point depression and molality to calculate the molar mass of biphenyl. Observations indicated that the freezing point decreased as more biphenyl was added to cyclohexane, which is consistent with colligative property principles where addition of solute decreases vapor pressure and consequently lowers the freezing point. The experimentally determined molar mass of biphenyl was 142.4 grams per mole, with a percent error of 7.5%.

Introduction: This laboratory focused on calculating the molar mass of biphenyl utilizing freezing point depression, a colligative property. Colligative properties are dependent on the number of solute particles in a solvent rather than their identity, and include boiling point elevation and freezing point depression. Specifically, adding a solute such as biphenyl to cyclohexane reduces the vapor pressure of the solvent and depresses its freezing point. The main goal was to quantify this depression to deduce the molar mass of biphenyl. The formula \(\Delta T = K_f \times m\) relates the freezing point depression (\(\Delta T\)) to the molality (m) of the solution, with \(K_f\) being the cryoscopic constant characteristic of the solvent.

Discussion: The experiment involved four trials, initially measuring the freezing point of pure cyclohexane using a temperature probe and Logger Pro software. Subsequently, biphenyl was added to cyclohexane in two additional tubes to create solutions with varying concentrations. The depression in the freezing point was observed to increase with higher biphenyl concentrations. Calculations revealed a molar mass of approximately 142.5 grams per mole for biphenyl, slightly less than the literature value, possibly due to experimental errors. Such errors may have arisen from inaccurate measurements of solute or solvent mass, loss of biphenyl during transfer, or temperature measurement inaccuracies. These factors could lower the calculated molar mass compared to accepted values.

Questions: In cases where students inadvertently lost some biphenyl during transfer, the measured molar mass would likely be underestimated. The missing mass would result in a lower calculated molar mass because the amount of solute used in the solution would be less than intended, affecting the depression observed and the resulting calculations. Accurate measurement of all reactants is essential for precise determination of molar mass using colligative properties.

Sample Paper For Above instruction

The determination of molar mass via freezing point depression demonstrates the practical application of colligative properties in analytical chemistry. In this experiment, cyclohexane served as the solvent, and biphenyl was the solute used to induce depression in the freezing point, thus enabling the calculation of biphenyl's molar mass. The fundamental principle relies on the colligative property that the addition of solute particles in a solvent lowers the freezing point proportionally to the molality of the solution.

From a theoretical perspective, the freezing point depression (\(\Delta T\)) is directly proportional to the molality (m) of the solution, expressed as \(\Delta T = K_f \times m\), where \(K_f\) is the cryoscopic constant specific to cyclohexane (1.97 °C·kg/mol). This relationship presumes ideal solution behavior and negligible ion pairing or association of the solute molecules, assumptions generally valid at low solute concentrations. In practice, accurate measurements are critical, and therefore, precise temperature readings and careful preparation of solutions are required to minimize experimental errors.

The experimental procedure involved initially measuring the freezing point of pure cyclohexane to establish a baseline. The solutions were prepared by dissolving known amounts of biphenyl into cyclohexane, with concentrations varying across the trials. The freezing points of these solutions were then determined using a temperature probe connected to Logger Pro software, which enabled accurate tracking of temperature changes during the freezing process. The decreased freezing points were recorded, and the differences from pure cyclohexane’s freezing point were used to calculate the molality and, subsequently, the molar mass of biphenyl.

The results obtained indicated a molar mass of approximately 142.5 g/mol, close but slightly lower than the published value of 154.2 g/mol. The deviation may have been caused by experimental errors such as loss of biphenyl during transfer, inaccurate weighing, or temperature measurement inaccuracies. Despite this discrepancy, the results support the methodology and theoretical framework of colligative property-based molar mass determination.

It is important to recognize the potential sources of error in such experiments. Loss of biphenyl during transfer is a common issue that can significantly impact the molar mass calculation, typically resulting in an underestimated value. Accurate transfer techniques, precise weighing, and consistent temperature measurement are critical for improving the reliability of the results. Future experiments could utilize more sophisticated analytical equipment, such as high-precision balances and more sensitive temperature probes, to enhance accuracy.

In conclusion, freezing point depression offers a powerful means to determine the molar mass of unknown substances. The experiment exemplifies the interplay between thermodynamic principles and practical laboratory techniques, reinforcing the importance of meticulous experimental procedures in obtaining valid scientific data. Despite minor deviations, the findings were consistent with theoretical expectations, underscoring the value of colligative properties in chemical analysis and characterization.

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