lec10_whc - Last Lecture Colligative Properties Comparison...

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Last Lecture • Colligative Properties • Comparison of Raoult’s and Henry's laws
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Earlier: Raoult’s law of Vapor Pressures Nonvolatile solute: Raoult’s Law still applies to solvent P A = x A, liq A Mole fraction of water in A must be greater (i.e. more dilute) Æ vapor pressure above A is greater (Raoult) Water vaporizes from A and condenses in B until vapor pressure above each is equal.
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Osmosis – A Colligative Property • Flow of solvent through a semi-permeable membrane – Driven by concentration differences Only water can pass through membrane Water moves from pure (higher mole ratio of water) to sucrose solution π π V = nRT π = RT n V = M RT This flow can be stopped by applying pressure! Osmotic pressure π – pressure to stop solvent flow Sucrose: C 12 H 22 O 11 Osmometry : determine molar mass from osmotic pressure Courtesy: www.the-aps.org
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A 0.10 g sample of a polymer dissolved in 100 mL of toluene has an osmotic pressure of 5.4 Torr at 20ºC. What is the molar mass of the polymer? Example 13G – Osmotic Pressure
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Solution 13G – Osmotic Pressure atm atm Hg mm Hg mm 0071 . 0 / 760 4 . 5 = = π It is convenient to first change to atm units:
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Solution 13G – Osmotic Pressure atm atm Hg mm Hg mm 0071 . 0 / 760 4 . 5 = = π It is convenient to first change to atm units: () mass polymer Molar mass polymer RT M RT V    = × V mass RT polymer mass Molar × = ) ( mol g L g atm K mol K
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lec10_whc - Last Lecture Colligative Properties Comparison...

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