December 14, 2012
Molecular Mass by Freezing Point Depression
Objective: Determine the molecular mass of an unknown substance by finding the change in freezing point of a solvent when the unknown substance is dissolved in it. Qualitative Data:
Cetyl –Alcohol (g)
Unknown substance (g)
Melting Temperature 1
Determine ___ for the solution of cetyl alcohol and of the unknown substance in BHT. 2.)
Calculate the molality of the cetyl alcohol solution and use it to calculate the value of the freezing point depression constant,____, for BHT. 3.)
Use the calculated value of ___ to find the molecular mass of the unknown solute.
Give a definition of colligative properties.
Draw a phase diagram of a pure substance, and show how addition of a solute affects this diagram. 3.
What is the least precise measurement? How does this limit your significant digits? 4.
Why is it advantageous to choose a solvent that has a large value for ____? 5.
Explain why the pure solvent shows a level horizontal curve as solidification occurs, but the curve for the solution slopes downward slightly.
I answered the objective because I was able to determine the molecular mass of the unknown substance by finding the change in freezing point of BHT when the unknown substance was dissolved in it. I learned how to apply __fp and molality. Two errors that could’ve occurred were timing the freezing point correctly and we might not have had one centimeter in the capillary tube. We didn’t know that we were supposed to stop timing after it started to become transparent, so our timings were all from when the whole centimeter of solution was transparent (except for trial 2). This would have affected our results by making our melting point temperature higher than it really should be. Since we didn’t have...
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