Freezing Point Depression

1
Freezing Point Depression
2
Experimental Objective

• Determine the freezing point depression constant,
  Kb, for the solvent phenyl salicylate
• Determine the molar mass of an unknown solute

3
Freezing point depression constant of phenyl
salicylate

• Determine freezing point of pure phenyl
  salicylate
• Add known quantity of naphthalene to pure phenyl
  salicylate
• Determine freezing point of solution
• Calculate Kf from data
• DTf Kfm

4
Molar mass of unknown solute

• Add known mass of unknown solute to pure phenyl
  salicylate
• Determine freezing point of solution
• Calculate molality of solution
• DTf Kfm
• Calculate molar mass of unknown solute

5
Experimental Method

• Calibrate Thermistor
• Determine freezing point of pure phenyl
  salicylate
• Determine freezing point of phenyl
  salicylate/naphthalene solution
• Determine freezing point of phenyl
  salicylate/unknown solution

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Calibrate Thermistor

• Two point calibration
• Ice bath
• Hot water (70-80 oC)
• Standard - thermometer

7
Freezing point of pure Phenyl Salicylate

• Known mass of phenyl salicylate
• Heat phenyl salicylate in hot water bath to
  60-65oC
• Cool to below freezing point
• Larger test tube, utility clamp, set-up
• Stir CONSTANTLY (rest arm on benchtop)
• Monitor temperature using LabWorks

8
Freezing point of Phenyl Salicylate/Napthalene
solution

• Heat phenyl salicylate in hot water bath to
  60-65oC
• Add known mass of napthalene to pure phenyl
  salicylate
• DO NOT rinse stirrer or thermistor
• Account for all napthalene
• Cool to below freezing point
• Larger test tube, utility clamp set-up
• Stir CONSTANTLY (rest arm on benchtop)
• Monitor temperature using LabWorks

9
Freezing point of Phenyl Salicylate/unknown
solute solution

• Obtain second sample of pure phenyl salicylate
• Heat phenyl salicylate in hot water bath to
  60-65oC
• Add known mass of unknown to pure phenyl
  salicylate
• Thoroughly clean stirrer and thermistor with
  hexane
• Account for all unknown solute
• Cool to below freezing point
• Larger test tube, utility clamp set-up
• Stir CONSTANTLY (rest arm on benchtop)
• Monitor temperature using LabWorks

10
Monitor Temperature using LabWorks

• Connect thermistor at connection 1
• Two point calibration of thermistor using
  thermometer as standard
• Solvent and solutions will supercool
• Freezing point is the plateau temperature
• Identify Tf from data (not graph)

11
Waste Disposal

• Heat solutions to above 65oC (completely liquify)
• Quickly pour liquified solution into organic
  waste liquid waste.
• Re-melt if necessary to transfer ALL of the
  solution into the liquid waste
• Rinse thermistor and stirrer with hexane after
  second and third freezing point determinations.

12
Data Analysis Determination of phenyl
salicylate freezing point depression constant, Kf
DTf Kf
m

• DTf Tf, pure phenyl salicylate- Tf, phenyl
  salicylate/napthalene solution
• Kf DTf /m Freezing point depression constant
  of phenyl salicylate

13
Data Analysis Determination of the molar mass
of the unknown solute
DTf Kf m

• DTf Tf, pure phenyl salicylate- Tf, phenyl
  salicylate/unknown solution
• Kf Freezing point depression constant of phenyl
  salicylate
• m DTf / Kf molality of unknown