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Freezing Point of a Solution of an Unknown Substance

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Add 2g of the unknown solid #1 and 10 mL of water to the test tube. the constant temperature bath is still set to dry ice. Drag and drop the constant temperature bath onto the test tube containing the solution.
Repeat the measurement of the freezing point with unknown solid #2 using a new clean test tube. As with solid #1 use 2g of solute and 10 mL of water.

1. Record the measured freezing point of pure water. 0 C
2. Record the freezing points and freezing point depressions, Td, of:
(a) unknown #1: -2.06 C
(b) unknown #2: -4.40 C

3. For each unknown solution, record the mass of water, the mass of the unknown compound and calculate the molality.
(a) unknown #1
(b) unknown #2

4. Calculate the molecular weight, MW, in g/mole for each unknown.
(a) unknown #1
(b) unknown #2

5. If you are told that one of the unknowns is glucose, C6HO6, with a molecular weight of 180.16 g/mole, which would you say it is?

6. What effect would each of the following have on the calculated molecular weight of an unknown? Would the calculated value be higher or lower than the actual value? Explain your answers.
(a) Some of the unknown does not dissolve.
(b) The thermometer reads 0.63°C higher than it should over the whole temperature range.
(c) The test tube is not dry (and has water drops inside) before the solutions are made up.

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1. Record the measured freezing point of pure water. 0 C (ANSWERED)
2. Record the freezing points and freezing point depressions, Td, of:
(a) unknown #1: -2.06 C (ANSWERED)
(b) unknown #2: -4.40 C (ANSWERED)

3. For each unknown solution, record the mass of water, the mass of the unknown compound and calculate the molality.
(a) unknown #1

Td = k(f) m

Td = change in freezing point = T(initial) - T(final) = 0 C - -2.06 C = 2.06 C
k(f) = molal freezing constant for water = 1.86 C/kg/mol
m = molality of the solution.

From this calculation we will find molality. The k(f) value is a constant value for water and can be found in any Chemistry textbook (or from http://www.cliffsnotes.com/study_guide/Freezing-and-Boiling-Points.topicArticleId-21729,articleId-21699.html). The change in temperature is always set up to be a positive value, because we are strictly looking at a DIFFERENCE in temperature.

Td = k(f) m
m = Td/k(f) = 2.06 C/(1.86 C/kg/mol) = 1.11 mol/kg

Because we divided by kg/mol, we end up with mol/kg as units. The C cancels.

The molality of unknown #1 is 1.11 mol/kg

(b) unknown #2

Td = k(f) m

Td = change in freezing point = T(initial) - T(final) = ...

Solution Summary

Freezing points are used to determine the molecular weight of solutes. A colligative property (freezing point depression) is employed in this calculation. All work is shown and explained.

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See Also This Related BrainMass Solution

Characteristic Properties - Melting and Boiling Points

Procedure 1

1. Take a Constant Temperature Bath from the Tools shelf and place it on the workbench.

2. Open the Properties window and click on the Constant Temperature Bath. Set the bath temperature to "dry ice".

3. Take a clean test tube from the Glassware shelf and place it on the workbench.

4. Fill the test tube with 5 mL of water.

5. Take a thermometer from the Tools shelf and drop it on the test tube.

6. Take the bath and drop it on the test tube, thereby "immersing" the test tube in the bath.

7. Open the Data window and click on the test tube. As the water freezes, you will see the moles of liquid decrease and the moles of solid increase.

8. Remove the test tube from the dry ice bath and use the Data window to verify that the moles of solid decreases (the ice is melting) and that the moles of the liquid water is increasing.

9. Verify that the temperature is stable during melting and record the temperature to the nearest 0.1 degrees. This is the melting point of the water.

10. Take a Bunsen burner from the Tools shelf and drop it directly on the test tube containing water. Open the Properties window, turn the Bunsen burner on and set the flame to low.

11. Click on the test tube so that the data window will display its contents.

12. Watch the thermometer and the Data window. When the water begins to boil, the temperature will remain at a constant value. You will see the vapor portion of the water escaping from the open test tube. It will NOT be listed in the Data window. However, the Data window will show the liquid decreasing. This is the boiling point of water.

13. Remove the test tube from the burner.

Procedure 2 ( click to view assignments for this procedure )

1. You will now repeat the procedures to determine the melting and boiling points for 4 unknown substances. In their liquid phase, they look like water and are hard to tell apart. However, their characteristic melting and boiling points will allow you to identify them.

2. Take a clean test tube from the Glassware shelf and place it on the workbench. Fill it with 5 mL of the first unknown liquid (L1).

3. Take a thermometer from the Tools shelf and drop it onto the test tube. Make sure that it connects to the test tube and displays room temperature (21C).

4. As with the water, measure the melting temperature of the substance by first immersing the test tube in the dry ice bath until some of the substance is frozen. Then remove the test tube and allow the frozen portion to begin melting before recording the melting temperature.

5. Measure the substance's boiling temperature heating the test tube with the Bunsen burner. Record the boiling temperature when the boiling temperature is constant while the substance is boiling out of the test tube.

6. Record the melting and boiling points for the unknown substance.

7. Repeat this procedure for the other three unknown substances.

Assignment 1 of Procedure 1

1. Record the melting point of water in degrees Celsius:

2. Record the boiling point of water in degrees Celsius:

3. Define the following terms:

(a) pure substance:

(b) melting point:

(c) boiling point:

(Enter Assignment Report)

Assignment 1 of Procedure 2

As we said above, the unknown liquids are clear and could be mistaken for water, but they can be identified as four organic solvents by their melting points (MP) and boiling points (BP). The literature values for these solvents are as follows:

acetonitrile (C₂H₃N): MP = -45.5C, BP = 77.3C
benzene (C₆H₆): MP = 6.0C, BP = 80.0C
carbon tetrachloride (CCl₄): MP = -23.0C, BP = 77.0C
cyclohexane (C₆H₁₂): MP = 7.0C, BP = 81.0C

1. Compare your data for the unknown liquids with the reference data above. Identify each of the four unknown liquids and explain how you reached your conclusions. Remember, experimental error may cause slight differences in some values.

2. For each unknown liquid, record the following:

(a) the measured and literature values of the melting and boiling points:

(b) the percent deviation of each substance, given by:

percent deviation = |(measured value) - (literature value)| / |(literature value)| * 100%

3. Discuss the possible reasons for the discrepancies between the experimental results and the literature values.

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