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ChemistryChemistry

TitleTitle

Determination of Molar Mass of an Unknown by Freezing Point Depression

Joe E. Student

Partners: Ima Dude and Fairly Shallow

01-11-02 

PurposePurpose

The molar mass of an unknown substance will be determined by freezing point depression in a solution of benzophenone. Using the known freezing point depression constant of 9.80 °C/m for benzophenone, the molality of the mixture containing the unknown can be determined using the equation ΔTf = kf m.

Experimental SetupExperimental Setup

Ring & Stand

Temperature Probe

Reaction Tube

Water Bath

DataData

Cooling Data of Pure Benzophenone Sample

Cooling Data of Benzophenone, Unknown Mixture

Trial 1 Trial 2 Trial 1 Trial 2

Elapsed Time (minutes)

Temperature ( °C )

0 55.0 55.0 55.0 55.0

1 50.5 50.2 50.0 50.2

2 47.5 46.5 45.3 45.0

3 48.2b 44.8 43.3 42.8

4 48.2 48.0b 41.5 40.3

5 48.2 48.2 39.0 37.5

6 48.2 48.2 38.0 35.1

7 48.2 48.2 37.0 34.2

8 48.2 48.2 35.8 34.5

9 48.2 48.2 34.9 38.2b

10 48.2 48.0e 33.7 38.2

11 47.8e 47.2 34.8 38.0

12 47.1 46.4 37.3b 37.5

13 46.5 38.2 37.5e

14 38.0 36.1

15 38.2 35.6

16 37.8 35.1

17 37.5 34.0

18

b - Freezing begins e - Freezing ends

37.0e

19 36.5

20 36.1

AnalysisAnalysisCalculationsMass of Benzophenone Sample • Mass of Benzophenone Sample = Mass Test Tube and Benzophenone - Mass Test Tube • Mass of Benzophenone Sample = 48.764 g - 38.362 g• Mass of Benzophenone Sample = 10.402 g

Mass of Unknown Solute• Mass of Unknown Solute = Mass Test Tube, Benzophenone, and Unknown - Mass Test Tube and Benzophenone • Mass of Unknown Solute = 49.649 g - 48.764 g • Mass of Unknown Solute = 0.985 g

Freezing Point Depression (ΔΔTf)• Freezing Point Depression = Benzophenone Freezing Point - Mixture Freezing Point • ΔTf = 48.2 °C - 38.0 °C • ΔTf = 10.2 °C

Molality of the Mixture (m)• ΔTf = kf m • m = ΔTf / kf• m = 10.2 °C / 9.80 °C/m• m = 1.04 moles solute / kg solvent

Moles of Unknown in Mixture• Moles of Unknown = Mass Benzophenone x molality of solution • Moles of Unknown = 10.402 g x 1.04 moles unknown / 1000 g benzophenone• Moles of Unknown = 0.0108 moles unknown

Molar Mass of Unknown• Molar Mass of Unknown = Mass Unknown / Moles Unknown • Molar Mass of Unknown = 0.985 g Unknown / 0.0108 moles Unknown• Molar Mass of Unknown = 91.1 g / mole

AnalysisAnalysis

ConclusionConclusionThis experiment used freezing point depression in a solution of benzophenone to determine the molar mass of an unknown substance. When a solute is dissolved in the pure liquid, and the temperature is dropped to the freezing point of the pure liquid, the liquid will not freeze. The solute particles disrupt the liquid molecules from settling down into an orderly solid array.  The system must get colder, & lose more energy, before the solvent freezes. By using the equation for freezing point depression and the number of particles in solution the molar mass of the unknown solute was found to be 91.1 g / mole. The accuracy of the calculated molar mass is limited by the accuracy of the temperature and mass measurements, however, far more significant errors arise as a result of visually estimating the beginning and ending points for freezing. The accepted value is given as 98.5 g/mole, 7.5 g/mole greater than our calculated molar mass. This would be an error of almost 10% by the equation.