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Video of freezing point depression lab

The video below presents the major steps for determining the freezing point depression of a pure substance. At the end of the video, a sample calculation is presented for determining the molar mass of the unknown. There are some differences between this lab and our lab, however. They are listed next:

• The video examines the freezing point depression for cyclohexane. In our lab, we will consider freezing point depression of lauric acid.
• Cyclohexane is a liquid at room temperature, whereas lauric acid is a solid at room temperature. In our lab, the amount of lauric acid is measured by taking its mass on an analytical balance.
• Since cyclohexane is a liquid at room temperature, its freezing point is determined by placing cyclohexane in an ice-water bath. For lauric acid, it must be heated first, then the substance freezes as it returns to room temperature.
• Our lab manual contains a section called Part II, which is optional. This video does not cover the optional procedure. Likewise, we will not consider Part II in this lab. We will only focus on Parts I and III.

As you are watching the video, record your qualitative observations in your lab notebook.

For your lab, you will be given data that you will analyze. The video given above is only example of what a similar lab experiment is like. For your data, you will need to assume that the lab was conducted as described in the lab manual.

Lab

Objective

• Using complete sentences, write the main objective(s) for this lab.

Part I

• The purpose of part I is to determine the normal freezing point of the pure solvent, lauric acid.
• The cooling curve for a pure substance should look like the graph below. We will only be looking at the boxed portion of the full cooling curve.

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• The freezing point of the pure substance should be the flat region of the curve that occurs following any observed supercooling. Sometimes supercooling is not observed.
• Diagram below shows example experimental data. There is a small amount of supercooling, followed by a somewhat flat region. This flat region represents the freezing temperature. The sloped region at the end represents the stage where the frozen substance cools after completely freezing.

Part III

• The purpose of part III is to determine the new freezing point when there is solute in dissolved in the solvent. The change in the freezing point of the pure solvent vs. the solution can be used to determine the molal concentration of solute particles that make up the solution.
• Adding the unknown causes the freezing point to be not as sharp. That is, freezing will occur over a wider range.
• The freezing point of the solution will be lower than the freezing point of the pure substance. This is the colligative property known as freezing point depression.
• A comparison of the cooling curves for the pure and impure substance is given in the diagram below.

• The diagram above exaggerates the differences for the segments of a typical cooling curve. For experimental data, the boundaries between the different sections is often more subtle. For example the section where the impure substance freezes and where the solid is cooling can be difficult to distinguish.
• Below is an example of experimental data for the freezing of a solution. Notice it is difficult to determine where freezing occurs and where freezing is complete (and the solid is cooling). However if you look closely, there is a section where the cooling curves levels off before temperature begins falling at a faster rate.

Cooling Curve of a Solution

• To estimate the freezing point for the solution, extrapolate a line of best fit for the liquid cooling section and the freezing section. The temperature where the lines intersect is the estimated freezing point for the solution (shown in the image below)

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Tasks:

This exercise asks you to complete the following

• • .
• Title Page
• Results and Calculations
  • Organize and present your data using the results that were provided to you.
    • Part 1:
      • the freezing points from each trial
      • the average freezing point for pure lauric acid
      • percent error from actual freezing point of lauric acid
    • Part III:
      • the mass of unknown and lauric acid used for each run
      • the molal concentration of unknown for each run
      • the individual freezing points from each trial
        • report the ΔT for each trial, but do not average them. It is inappropriate to average the ΔT for part III since they should be different depending on how much unknown was added.
      • the individual molar mass for the unknown, calculated from each run
      • the average molar mass for the unknown
      • the percent error for your molar mass, assuming your unknown is benzoic acid.
    • Show sample calculations for
      • molal concentration of unknown
      • moles of unknown
      • percent error
    • Create tables summarizing the major experimental data.
• Conclusion
  • Evaluate the objective(s) of the experiment and restate the freezing point and percent error of lauric acid, and the molar mass of the known and percent error.
• Post Lab Questions
• Spreadsheet download
• Provided Data