Determining Molecular Weight by Freezing Point Depression: A Laboratory Report
The determination of molecular weight is a fundamental aspect of chemistry, providing crucial insights into the composition and properties of substances. One common technique for this determination is the freezing point depression method, which exploits the colligative property of freezing point depression. This method relies on the principle that the freezing point of a solvent is lowered when a non-volatile solute is dissolved in it. The magnitude of this depression is directly proportional to the molality of the solution.
This laboratory report outlines the procedure and results of an experiment conducted to determine the molecular weight of an unknown solute using the freezing point depression method. We will analyze the data obtained and discuss potential sources of error that may have influenced the results.
Materials and Methods
The experiment was performed in a controlled laboratory setting with the following materials:
- Unknown solute (sample)
- Cyclohexane (solvent)
- Thermometer
- Beaker
- Stirring rod
- Ice bath
- Hot plate
- Graduated cylinder
Procedure:
-
Preparation of the Cyclohexane solution:
- Accurately weigh a specific amount of cyclohexane using a balance and record its mass.
- Transfer the cyclohexane to a clean and dry beaker.
- Add a measured amount of the unknown solute to the cyclohexane and dissolve it completely by stirring.
- Record the mass of the unknown solute added.
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Determining the Freezing Point of Pure Cyclohexane:
- Place the beaker containing pure cyclohexane in an ice bath.
- Stir continuously and monitor the temperature of the cyclohexane using the thermometer.
- Record the temperature at which the cyclohexane starts to solidify and remains constant for a few seconds. This temperature is the freezing point of pure cyclohexane.
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Determining the Freezing Point of the Cyclohexane Solution:
- Repeat step 2 using the prepared cyclohexane solution containing the unknown solute.
- Record the freezing point of the solution.
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Calculations:
- Calculate the molality of the solution using the formula:
Molality (m) = (Mass of solute (g) / Molecular weight of solute (g/mol)) / (Mass of solvent (kg)) - Calculate the freezing point depression (ΔTf) using the formula:
ΔTf = Freezing point of pure cyclohexane - Freezing point of solution - Using the freezing point depression constant (Kf) of cyclohexane (20.0 °C/m), calculate the molecular weight of the unknown solute using the formula:
Molecular weight of solute = (Mass of solute (g) / ΔTf (°C)) * Kf (°C/m)
- Calculate the molality of the solution using the formula:
Results and Discussion
Table 1: Experimental Data
| Parameter | Value |
|---|---|
| Mass of cyclohexane (g) | |
| Mass of unknown solute (g) | |
| Freezing point of pure cyclohexane (°C) | |
| Freezing point of solution (°C) |
Calculations:
- Molality (m) =
- Freezing point depression (ΔTf) =
- Molecular weight of unknown solute =
Discussion:
The experimentally determined molecular weight of the unknown solute was found to be g/mol. This value can be compared to the theoretical molecular weight of the unknown solute, if it is known. The difference between the experimental and theoretical values provides an indication of the accuracy of the experiment.
Several factors can contribute to errors in the determination of the molecular weight using the freezing point depression method:
- Impurities in the solvent: The presence of impurities in the cyclohexane can affect its freezing point and introduce errors in the calculation of the molecular weight.
- Incomplete dissolution of the solute: If the unknown solute does not completely dissolve in the cyclohexane, the concentration of the solution will be lower than expected, leading to an underestimation of the molecular weight.
- Errors in temperature measurement: Inaccurate temperature readings can significantly affect the calculated freezing point depression and the molecular weight.
- Heat loss or gain during the experiment: The transfer of heat from the surroundings can affect the temperature of the solution and introduce errors in the freezing point measurements.
Conclusion
The freezing point depression method proved to be a useful technique for determining the molecular weight of an unknown solute. However, it is important to be aware of potential sources of error and take appropriate precautions to minimize their impact on the results. Careful attention to experimental details and the use of precise measurements are crucial for obtaining accurate results. By understanding the principles behind the method and identifying potential error sources, we can improve the reliability and accuracy of our molecular weight determinations.