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Determining Freezing Point Depression Constant for Acetic Acid

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Introduction

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: vapor pressure; boiling point; depression of freezing point and osmotic pressure. 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. ...read more.

Middle

Placed test tube in ice water and stirred until solution was super-cooled and could freeze. Recorded temperature after super-cooling was complete and temperature had leveled. 6. Input data into formula Kf = [Tf(MM solute)(kg solvent)]/g solute and solve for Kf. 7. Repeat twice and find average Kf value. Data and Calculations: Experimentally determined freezing point temperature for pure acetic acid: 14.73 degrees C. See attached graphs for thermistor data Attachment 1: untitled1 acetic acid freezing point Attachment 2: FP Acetic Trial #1 Attachment : FP Acetic Trial #2 Attachment 4: FP Acetic Trial #3 Kf for acetic acid: Trial #1 Trial #2 Trial #3 Mass of test tube and beaker 99.717g 99.721g 99.244g Mass of test tube, beaker & solvent 103.110g 103.486g 104.282g Mass of test tube, beaker, solvent & solute 103.365g 103.883g 104.665g Freezing temperature of solution 12.88C 12.92C 9.79C Calculated mass of solvent 3.393g 3.765g 5.042g Calculated mass of solvent (kilograms) .003393 .003765 .005042 Calculated mass of solute .255g .397g .383g Calculated change in Tf 1.85C 1.81C 1.79C Calculated Kf (degrees C/m) ...read more.

Conclusion

Trial #2 was thrown out and not used for the final calculation because this trial was an outlier; it may have had such bad results because the amount of solute was so large in the solution that it did not fully dissolve. So only a percent of the solute was able to affect the freezing point of the acetic acid resulting in a very off end result. The average calculated from the other trials was a freezing point depression constant (Kf) for acetic acid of 4.35. The known Kf for acetic acid is 3.9. The calculated average Kf was 3.9. This may have been caused because the first and third trial was slightly high and the second trial was very low. One possible cause of error was that the temperature may not have been at 25 degrees Celsius and that is what the accepted value was calculated for. Also the solution underwent supercooling so it may have not been its exact freezing points. Other modes of error could be slight discrepancies weighing the solute and solvent which would change results. The experimentally obtained K value had an 11% error so the lab was successful. ...read more.

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