Determining Freezing Point Depression Constant for Acetic Acid

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Noelle Carpenter        

-01736014

chem 132-3

July 27 2009

Project #1- Freezing Point Depression Constant for Acetic Acid

Introduction: 

        

        The purpose of this lab was to experimentally determine the colligative property; freezing point depression constant, for acetic acid. Colligative properties are properties of solutions that depend on the number of molecules in a given amount of solvent and not on properties such as size or mass of the molecules. Colligative properties include: ; ;  and . Freezing point is the equilibrium between a liquid and solid phase and is lowered by the presence of a solute compared to a pure solvent. The solute particles cannot enter the solid phase, hence, less molecules participate in the equilibrium so equilibrium is achieved at a lower temperature at which the rate of freezing becomes equal at the rate of solidifying. Because an impurity is being added the freezing temperature of the pure acetic acid would be higher than when acetylsalicylic acid is added to the solution, when the molecules of the solvent freeze they form crystals of pure solvent.  At the freezing point of pure solution there was a equilibrium of molecules joining and detaching from the solid but when a solute is added to the solution in the crystals remain as pure solvent so the solute cannot join.  The solute comes into to contact with the crystal because it is part of the solution.  When the solute is added the solvent cannot come into contact with the crystals and join the solid as fast as before because of the presence of the solute.  This means the freezing point is lowered. The freezing point depression constant can be solved for if the change in the freezing point after solute is added, mass of the solute and solvent in the solution and the molar mass of the solute are known.  Freezing point depression is used by mixing compounds and changing their properties by changing the concentration of solute to solvent. Freezing point depression is used in anti-freeze fluid, homemade ice cream and in technical applications to avoid freezing.

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Procedure

  1. MicroLAB is set up to measure the temperature change with temperature in Celsius on the Y axis and time in seconds on the X axis.
  2. A clean beaker and test tube were weighed and mass recorded
  3. a few mL acetic acid were added to test tube, weighed with beaker and mass was recorded.
  4. Add10 drops acetylsalicylic acid to test tube, weighed this and beaker and recorded mass.

Trial #1 acetylsalicylic acid weight = .255 Trial #2 acetylsalicylic acid weight = .397 Trial #3 acetylsalicylic acid weight = .383

  1. Placed test tube in ...

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