Molar Mass Determination by Freezing Point Depression

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Molar Mass Determination by Freezing Point
Depression

• Experiment 7

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7 Molar Mass Determination by Freezing Point
Depression

• Goal
• To find the molar mass of an unknown solid
• Method
• Determine freezing point depression constant for
  pure solvent (cyclohexane)
• Use the freezing point depression of a solution
  containing the solid to find the solutes molar
  mass

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Colligative Properties

• Solution properties differ from those of pure
  solvent
• Proportional to molality (concentration) of
  solute
• Vapor pressure reduction
• Freezing point depression
• Boiling point elevation
• Osmotic pressure
• Osmotic pressure Key in BIO 20

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Phase Change Freezing/Melting

• Liquid cools at constant rate

Temperature
Liquid starts freezing
More more liquid freezes
FP
Time
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Actual Freezing
Temperature
Rounded T not uniform throughout
FP
Time
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Phase Change Temperature

• Freezing point pure liquid Melting pointpure
  solid

?rates?l ?ratel?s
Pure liquid solvent (lo)
Pure solid solvent (so)
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Solute added

• Equilibrium at lower T so freezing point is LOWER
• Related to energy and disorder

?rates?l gt ?ratel?s
Solvent(l) solute
Pure solid solvent (so)
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Energy and Entropy Changes

• Free energy

Tf
Tfo
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Colligative Property

• Magnitude of freezing point depression

Depends on concentration of solute (not identity)
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Freezing point depression

• Kf molal freezing point constant
• specific to solvent
• Units C
• molal
• What is molal?

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Molality

• Concentration in molality, m

Independent of solution volume (V varies with T)
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Freezing point depression
Using
And

• Substitution gives

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Kf and Kb for some solvents

• Freezing point is lower
• Boiling point is higher

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Example

• 0.500 g of an unknown organic compound is
  dissolved in 10.0 g benzene. The freezing point
  lowers by 2.97C.
• Calculate the molecular mass of the unknown..
• Kf, benzene 5.12 C/m

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Overview

• 1) From temp. vs. time plots, find freezing
  points for
• pure solvent (cyclohexane) and
• 0.05, 0.10, 0.15 m p-dichlorobenzene solutions
• 2) Using data from (1), plot DTf vs. molality to
  find Kf
• 3) Find DTf of a solution containing unknown
  solid
• 4) Use Kf from (2) and DTf from (3) to find
  MMsolid

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Part 1 Determination of Cyclohexanes Kf

• Prepare samples
• Pure cyclohexane r20o 0.779 g/mL
• 0.05 m, 0.10 m, 0.15 m p-dichlorobenzene
• MM 147.00 g/mol
• DO NOT weigh out in advance

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Part 1 Determination of Cyclohexanes Kf

• Each sample
• Immerse in ice bath
• _at_ T13C
• record temp. and time T, t
• Plot T vs. t ? find Tf and DTf Tf, DTf
• Plot Tf vs. molality
• Find Kf from slope Kf
• Compare to lit. value (20.2oC/m)

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Examples Temperature vs. Time

• Tfo 6.5oC
• Tf 2.4oC

You will have separate plots!
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Ideal DTf vs. molality
Slope Kf
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Example Data

• Experimental Kf
• 19.9oC/m
• error 1.5
• Theoretical Kf
• 20.2oC/m

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Part 2 MM Detemination if Unknown Solid

• Pure solvent
• Immerse in ice bath
• _at_ T13C, record temp. and time T, t
• With 0.37 g unknown
• Dissolve perform trial(s)
• _at_ T13C, record temp. and time T, t
• Find moles n and n
• MM of unknown solid MM

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Example Unknown Data

• Tfo 6.5oC
• Tf 4.5oC

You will have separate plots!
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Example Unknown MM Data
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Report

• Abstract
• Results
• Sample calculations including
• Kf determination
• Molar Mass determination (ID)
• At least 6 graphs tables
• Temp. vs. time
• DTf vs. molality p-dichlorobenzenes and
  unknown
• Discussion/review questions

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Properties of cyclohexane

• Property Value
• Freezing point 6.50C
• Boiling point 80.72C
• Kf 20.2C/molal
• P at 25C 99.0 torr