laboratory 2 molecular weight by freezing point depression lab report

Julian Assange

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07-09-2024

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Introduction

In the world of chemistry, understanding the molecular weight of a substance is essential for various applications, from drug development to material science. One of the elegant methods to determine the molecular weight of a solute is through freezing point depression. This technique takes advantage of the principle that the freezing point of a solvent decreases when a solute is dissolved in it. In this lab report, we will discuss the methodology, results, and implications of using freezing point depression to calculate molecular weight.

Objective

The primary objective of this laboratory experiment is to determine the molecular weight of an unknown solute using the freezing point depression method.

Theory

When a solute is dissolved in a solvent, the freezing point of the resulting solution will be lower than that of the pure solvent. This phenomenon can be described by the formula:

[ \Delta T_f = K_f \cdot m ]

Where:

• ( \Delta T_f ) = change in freezing point (°C)
• ( K_f ) = freezing point depression constant of the solvent (°C kg/mol)
• ( m ) = molality of the solution (mol/kg)

From the above equation, the molality can be rearranged to find the molecular weight (M) of the solute:

[ M = \frac{g}{m \cdot \text{kg solvent}} ]

Where:

• ( g ) = mass of the solute (g)

Materials

• Pure solvent (water)
• Unknown solute
• Analytical balance
• Thermometer
• Ice bath
• Stirring rod
• Beakers

Procedure

1. Preparation of the Solution:

   • Measure a specific mass of the unknown solute using the analytical balance.
   • Dissolve the solute in a known mass of the solvent (water) in a beaker.

2. Determine the Freezing Point of Pure Solvent:

   • Place the thermometer in the pure solvent and allow it to reach a stable temperature. Record this temperature as ( T_{f\text{,solvent}} ).

3. Determine the Freezing Point of the Solution:

   • Place the beaker containing the solution in an ice bath.
   • Stir gently until the temperature stabilizes at the new freezing point. Record this temperature as ( T_{f\text{,solution}} ).

4. Calculate Freezing Point Depression:

   • Calculate the change in freezing point using the formula: [ \Delta T_f = T_{f\text{,solvent}} - T_{f\text{,solution}} ]

5. Calculate Molality:

   • Determine the molality of the solution using the formula: [ m = \frac{\Delta T_f}{K_f} ] (Assuming ( K_f ) for water = 1.86 °C kg/mol)

6. Calculate Molecular Weight:

   • Finally, determine the molecular weight of the unknown solute using: [ M = \frac{g}{m \cdot \text{kg solvent}} ]

Results

Note: Replace the placeholders with actual data obtained during the experiment.

Discussion

The results obtained from this experiment can be used to determine the molecular weight of the unknown solute accurately. The values of ( K_f ) and the measurements must be precise as small errors can lead to significant discrepancies in the final result. Furthermore, if the calculated molecular weight does not align with expected values, it may indicate experimental errors or impurities in the solute.

Common Sources of Error:

• Impurities in the solute.
• Inaccurate temperature measurements.
• Failure to account for evaporation of the solvent.

Conclusion

The freezing point depression method is a practical and insightful technique to determine the molecular weight of unknown substances. This laboratory experiment not only reinforces the principles of colligative properties but also enhances our practical skills in laboratory techniques and calculations.

References

• Chemistry textbooks on colligative properties.
• Lab manuals outlining freezing point depression experiments.

Feel free to explore more about colligative properties and their applications in chemistry through the following links:

• Colligative Properties Overview
• Applications of Freezing Point Depression

This lab report template serves as a guide to effectively present your findings and methodologies in laboratory settings.