Molar Mass by Freezing Point Depression

Lab Name: Molar Mass by Freezing Point Depression Researcher: Isabella Cuenco Lab Start Date: November 9, 2012 Lab Completion Date: November 9, 2012
INTRODUCTION
Purpose: The purpose of the lab is to find the molar mass of an unknown substance by measuring the freezing point depression of a solution of the unknown substance and BHT. Hypothesis: If the freezing point depression of a solution of an unknown substance and BHT is measured, the molar mass of an unknown substance can be found.

Pre-Lab Questions:

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Determine the freezing point depression 53. 02 – 50. 78 = 2. 24 °C
Calculate the molar mass of the unknown substance 7.
°C/m X 2. 04 g (solute) X 24. 8 g (solvent) X 2. 24 °C = 260. 0 g molar mass = 260. g

What are colligative properties? Colligative properties are properties of a solution that change when the condition of the solution changes.
PROCEDURE Part B

Set up a Bunsen Burner, ring stand and clamp, as shown in picture below.
Fill a beaker with 100 mL of water
Place beaker on ring stand, and light burner to test that blue of flame is hitting the bottom of the beaker; once it is, turn burner off.
Using a mortar and pestle, crush 0. 5 g of BHT.
Pack the BHT into a small capillary tube.
Using a rubber band, fasten the capillary tube to a thermometer, ensuring the bottom of the tube lines up with the thermometer bottom.
Clamp the thermometer/tube, ensuring the thermometer and tube are in the water. 8. Begin to heat the water and observe the tube.
Once the BHT has melted (turned from white powder to clear), turn off the heat, and record the temperature at which the BHT melted.
Once cool, dispose of the BHT and tube. 11. Using a mortar and pestle, crush  1 g of cetyl alcohol.
Using a mortar and pestle, crush  5 g of BHT.
Pack the BHT and cetyl alcohol into a small capillary tube.
Repeat steps 6-10 for the BHT and cetyl alcohol.

RESULTS (DATA & OBSERVATIONS):
Part A (Sample Data given):
Trial #1Trial #2 Mass of empty test tube #1, g18. 235 g Mass of test tube #1 + BHT, g26. 292 g Mass of BHT, g8. 057 g Mass of weighing paper, g0. 221 g Mass of weighing paper + cetyl alcohol, g1. 236 g Mass of cetyl alcohol, g1. 15 g Mass of empty test tube #2, g18. 689 g Mass of test tube #2 + BHT, g26. 679 g Mass of BHT, g7. 990 g Mass of unknown, g1. 656 g Temperature in ? C: Time, in secondsPure BHTBHT + cetyl alcoholBHT + unknown 085. 085. 576. 8 2080. 084. 974. 7 4075. 881. 674. 5 6072. 078. 672. 2 8069. 076. 369. 8 10068. 873. 567. 8 12069. 072. 065. 9 14068. 869. 764. 3 16068. 667. 462. 9 18068. 465. 561. 6 20068. 264. 260. 4 22063. 661. 1 24063. 861. 5 26063. 761. 6 30063. 561. 2 36060. 5 420 480 Part B: Melting Points: Pure BHT71. 9 ? C BHT + cetyl alcohol68. 5 ?
Masses:
BHTCetyl Alcohol Solution #1 – BHT + Cetyl Alcohol, g0. 5 g0. 1 g IV. ANALYSIS: Post- Lab Calculations Determine ? Tfp for the solution cetyl alcohol and of the unknown substance in BHT. Calculate the molality of the cetyl alcohol solution and use it to determine the value of the freezing point depression constant, kfp, for BHT. Use the calculated value of kfp, along with the masses of the unknown solute and BHT, to find the molar mass of the unknown solute. molality of cetyl alcohol solution = 0. 5 m kfp of BHT = 4. 0 ? C/m molar mass of unknown solute = 240 g/mol
Post-Lab Questions 1. The following errors occurred when the above experiment was carried out. How would each affect the calculated molar mass of the solute (too high, too low, no effect)? Explain your answers.

The thermometer used actually read 1. 4 ? C too high.
Some of the solvent was spilled before the solute was added.
Some of the solute was spilled after it was weighed and before it was added to the solvent.
Some of the solution was spilled after the solute and solvent were mied but before the freezing point was determined.

What was the least precise measurement in the experiment? How does this limit your significant digits?
Did the solutions show any evidence of supercooling?
Why is it advantageous to choose a solvent that has a large value for Kfp?
Explain why the pure solvent shows a level horizontal curve as solidification occurs, but the curve for the solution slopes downward slightly.

CONCLUSION
When the freezing point depression of a solution of an unknown substance and BHT is measured, the molar mass of an unknown substance is found. The hypothesis

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