Determination of Molar Mass by Freezing Point Depression

AP Lab #8 Name

Date

Purpose: (brief statement of what you are attempting to do. These are similar to the objectives. )

Procedure: (A brief description of the method you are using. You may refer to the lab document for specific instructions, but you should include a brief statement of the method. DO not include lengthy, detailed directions. A person who understands chemistry should be able to read this section and know what you are doing. Include all chemicals used and the major equipment. )

PreLab:

  1. Why do you need to use the same thermometer to find the freezing point of water and the freezing point of the solution?
  2. Why is it important that the thermometer does not touch the sides or bottom of the test tube while you are recording temperatures?
  3. Why is it necessary to add salt to the ice bath in this experiment?

Results:

Observations: (General descriptions of visible appearances or changes that occur during the experiment, such as “table salt is a white, cube-shaped crystal which dissolves in water. (Qualitative))

Data: (Neatly arranged measured values listed in tabular form. The units of measurement MUST be included with the numerical values. The accuracy of the measurement can also be included as a range (+/-). Calculated answers that are derived by performing a simple mathematical operation can also be included in the data table. If the graphs are included, make the graphs an appropriate size. Label all axes and give each graph a title.)

Number of ions in one formula unit of the unknown solute
Mass of unknown solute (grams)
Volume of solvent (water mL)
Water / Solution
Time / Temperature / Time / Temperature

Calculations: (Show all calculations with formula and appropriate units on all numbers. Neatly demonstrate the math set-ups, including units. Label what is being calculated. –make it organized. Show error calculations where appropriate. If experiments are qualitative, this section may be omitted.)

  1. Plot cooling curves for pure water and the solution.
  2. Find the freezing point depression, ΔTf
  3. Calculate the number of moles of the unknown compound that were added to the distilled water.
  4. Calculate the molar mass of the unknown compound.
  5. Calculate the percentage error if the actual molar mass of the compound is 58.5 g/mol.

Conclusion: (Make a simple statement concerning what you can conclude form the experiment. Refer back to the purpose of the lab to write this section. (i.e. How was the purpose of the experiment fulfilled?))

Discussion of Theory: (In this section you should include such information as: What theory was demonstrated in this experiment (Include concepts used in the experiment)? What do the calculations show? Why does (or doesn’t) the experiment work? This section shows me that you understand the concepts used in the lab. Be detailed and ask if you need help! )

Experimental Sources of Error: (What are some specific sources of error, and how do they influence the data? Do they make the values obtained larger or smaller than they should be? Which measurement was the least precise? Instrumental error and human error exist in all experiments, and should not be mentioned as a source of error unless they caused a significant fault. Significant digits and mistakes in calculations are NOT a valid source of error. In writing this section it is sometimes helpful to ask yourself what you would do differently if you were to repeat the experiment and wanted to obtain better precision. If you can calculate percent error, do so and include in this section. )

Post Lab Questions: (Answer any questions included in the lab. Answer in such a way that the meaning of the question is obvious in your answer. )

  1. What are the measurements that must be taken to find the molar mass of an unknown solid using the freezing point depression method?
  2. When a student finished mixing the solution in the test tube (step 8), the fire alarm went off and the building was evacuated for a period of time. After returning to class, the student finished the experiment. How would the student’s calculated value for the number of moles of unknown solid be affected if several milliliters of water evaporated from the test tube while the student was out of the room? Justify your answer.
  3. The van’t Hoff factor is often a little less than the number of ions in one formula unit of an ionic compound due to ion pairing and ion clusters. Explain why this is.
  4. How would ion pairing and ion clusters in the solution affect the calculated value of the molar mass in this experiment?
  5. Suppose that the thermometer you used in this experiment consistently read 0.9°C below the actual temperature. What affect would this have on the experimentally determined value for the freezing point depression, ΔTf? Explain.
  6. Suppose that some of the unknown solute did not dissolve in the 20 mL of water due to saturation. How would this affect the calculated value of the molar mass? Explain.
  7. Rank the four liquids tested from most volatile to least volatile based on their observed temperature changes due to evaporation.
  8. Look up the boiling points of the four compounds tested in this experiment. Is there a relationship between the rate of evaporation of a liquid and its boiling point.

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