1M_07_Freezing

1M_07_Freezing - Molar Mass Determination by Freezing Point...

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Unformatted text preview: Molar Mass Determination by Freezing Point Depression Experiment 7 #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 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 Phase Change: Freezing/Melting L i q u i d c o o l s a t c o n s t a n t r a t e Temperature Time L i q u i d s t a r t s f r e e z i n g M o r e & m o r e l i q u i d f r e e z e s FP Actual Freezing Temperature Time R o u n d e d : T n o t u n i f o r m t h r o u g h o u t FP Phase Change Temperature Pure solid solvent (s o ) Pure liquid solvent (l o ) Freezing point pure liquid = Melting point pure solid rate s l = rate l s Solute added Pure solid solvent (s o ) Solvent (l) + solute Equilibrium at lower T so freezing point is LOWER Related to energy and disorder rate s l > rate l s Energy and Entropy Changes Free energy T f T f o Colligative Property Magnitude of freezing point depression: <- = f,solvent f,solution f T T T [ ] solute K T f f- = Depends on concentration of solute (not identity) Freezing point depression...
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1M_07_Freezing - Molar Mass Determination by Freezing Point...

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