Molar Mass by FP depression lab by nuhman10

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```									Molar Mass Determination by Freezing Point Depression
I. Purpose- To determine the molar mass of a solute by determining the freezing point of the pure solvent
and the solution. Using the freezing point depression you can find the molar mass of the solute.

T = kfmi                kf lauric acid = 3.900C/m
II. Procedure
Part One- Determining the Freezing Point of the Pure solvent (Lauric Acid)
1. Set up a water bath by placing 300 ml of water into a 400 ml beaker.
2. Place on a heating block and heat water until it is approximately 55 0C.
3. Obtain a test tube of hot Lauric Acid from your instructor.
4. Place a thermometer in the test tube.
5. Remove the beaker from the heating block.
6. Place the test tube in the water.
7. Monitor and record the temperature of the lauric acid.
8. Slightly stir the liquid to assure even cooling.
9. Record the freezing point – THE TEMPERATURE WILL REMAIN CONSTANT DURING THE
PHASE CHANGE. THAT IS THE FREEZING POINT.
10. Remove thermometer and rinse and dry.
11. Give the test tube back to your instructor.

Part Two- Determine the Freezing Point of the Solution of Benzoic Acid and Lauric Acid
1. Set up two water baths by placing 300 ml of water into two separate 400 ml beakers.
2. Place on a heating block and heat water until it is approximately 70 0C. Heat the other bath until it is
approximately 300C.
3. Measure approximately 8 grams of Lauric acid. Measure 1 gram of Benzoic acid (SET ASIDE FOR
LATER USE) Record exact masses.
4. Place Lauric acid into a test tube. Place the test tube into the water bath at 70 0C and allow it to melt.
When the entire sample is melted, add the benzoic acid, while stirring. Allow the solute to completely
dissolve.
5. Remove the test tube from the water bath at 70 0C and place it in cooler water bath.
6. Record temperature and time readings every 30 second until the solution reaches 350C
7. Graph temperature vs. time to determine the freezing point of the solution.

III. Data

1. Freezing point of pure solvent                              ____________0C

2. Mass of Lauric acid                 ___________ g           ___________ kg

3. Freezing point of solution                                  ___________ 0C

4. Tf                                                         ___________ 0C

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5. molality of solute                                   ___________ m

6. moles of solute                                      ____________moles

7. molar mass of benzoic acid                            _____________ g/mol

8. percent error                                     _______________%

Freezing point pure Lauric acid

Time (sec)
Temperature
(oC)

Freezing point Benzoic Acid/ Lauric acid Solution

Time (sec)
Temperature
(oC)

Data - Temperature vs Time graph of solution freezing point

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Discussion Questions
1. Why is a molality concentration scale used in the context of colligative properties and not the more
common concentration scale, molarity?

2. Assume that you have a solution of an unknown solute in cyclohexane. If the solution has a freezing
point depression of 9.50 0C, what is the molality of this solution? (The freezing point depression
constant for cyclohexane (Kf) is 20.4 0C/m).

3. Cyclohexanol, C6H11OH, is sometimes used as the solvent in molecular weight determination by
freezing point depression. If 0.253 g of benzoic acid, C6H5COOH, dissolved in 12.45 g of
cyclohexanol, lowered the freezing point of pure cyclohexanol by 6.55 0C, what is the molal freezing
point constant (Kf) of this solvent?

4. What is supercooling? How can it be minimized?

5.   Distinguish between non-volatile and volatile substances. Give two examples of each.

Conclusion- Write one

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