1. What are we looking to find in this experiment? 2. What will you measure in this experiement to determine the molar mass of your unknown solid? 3. How will you use the freezing point depression information to determine the molar mass of the unknown solid? 4. What theories or equations will allow you to determine the molar mass of the unknown solid? 5. What data will be collected in this experiment and how will it be used to determine the molar mass of the unknown solid? 6. Describe the important techniques that will be used to collect the data in this experiement? 7. What important assumptions are made about the unknown solid in order to determine its molar mass using freezing point depression? 8. What is the expected outcome of this experiment? What are some possible unexpected outcomes? 9. What is the scientific relationship between freezing point depression and the amount of solid that will allow us to correctly determine the molar mass? 10. What important variables need to be accounted for so that only the relationship between freezing point depression and the mass of the unknown solid is being measured? 11. What special safety precautions do you need to be aware of for this experiment?
EXPERIMENT #3: MOLECULAR WEIGHT DETERMINATION FROM FREEZING POINT DEPRESSION
After figuring out the identity of a volatile organic liquid in a recently vacated laboratory (see Project 2), you came upon another bottle of unknown compound with misplaced label, and again it is only known that it comes from a short list of compounds. Again, you are asked to figure out the identity of this unknown compound. Unfortunately this time the compound is not volatile therefore you cannot use Dumas method. You did some tests and found that this compound was soluble in cyclohexane, an organic solvent, which made you believe you could use freezing point depression to figure out the molecular weight of the unknown compound. Design an experiment and carry out this task.
1. Determine the molecular weight of the unknown compound using freezing point depression data.
Colligative properties are properties which depend on the number of solute particles dissolved in a solvent. These include boiling point elevation, freezing point depression, and vapor pressure lowering. When a solute is dissolved in a solvent the change in the colligative properties is directly proportional to the concentration of the solution. The freezing point depression for a given solvent is related to the molality of a solution by the equation,
ΔTf = iKfm (1)
where ΔTf is the freezing point depression, which is the difference between the freezing point of the solvent and the freezing point of the solution; Kf is the molal freezing point constant of a given solvent, m is the molality of the solution, and i is the vant’t Hoff factor, which measures the extent electrolytes dissociate in solution. For a nonelectrolyte compound, such as glucose, i is equal to 1. For NaCl, an electrolyte compound, the ideal value for i is 2, since each mole of NaCl contains one mole of Na+ ions and one mole of Cl- ions. Table 1-1 gives the freezing point of Kf values for some common solvents.
Table 1:. The Freezing points of molal freezing point depression values for some common solvents
Freezing Point, ºC
Therefore, if you dissolve a certain amount of the unknown compound in cyclohexane (known mass) and measure the freezing point change of cyclohexane, you can calculate the molality of the unknown compound in the solution which, together with the mass of the compound you used, will allow you to find out the molecular weight of the compound. Below is the cooling curve you will us to find the freezing point for this experiment.
Table 2: Cooling Curve for Cyclohexane
Freezing Point Depression for
Cyclohexane and the Solution
Figure 1: Freezing Point Depression of Cyclohexane and the Solution
Physical Properties of substances to consider:
General Chemistry Concepts: freezing point depression, colligative properties
Laboratory: Cleaning of glassware, measuring mass
Computer: apparatus setup using chemcketch, interpretation of data using excel
Special Apparatus and Chemicals:
Large test tube
Analysis of experimental data through calculations. (Report in table form)
Make the cooling curve for cyclohexane and the solvent using excel
From the graph determine the freezing point of cyclohexane and the solution Determine the freezing point depression of the solution
Determine the gram molecular weight of the solute Perform error analysis on the data collected
Analysis of data and calculations through discussion. (This belongs in the Discussion section in the lab
Refer back to the project. Did you answer the question?
Compare the freezing point depression of cyclohexane and the solution Explain how you were able to determine the molecular weight of the solute.