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5_60_lecture23

# 5_60_lecture23 - MIT OpenCourseWare http/ocw.mit.edu 5.60...

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MIT OpenCourseWare http://ocw.mit.edu 5.60 Thermodynamics & Kinetics Spring 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms .

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5.60 Spring 2008 Lecture #23 page 1 Colligative Properties These are properties of solutions in the dilute limit, where there is a solvent “A” and a solute “B” where n A >> n B . These properties are a direct result of µ A mix ( l ,T,p) < µ A pure ( l ,T,p) Use two measures of concentration: a. Mole Fraction: x B = n B /(n A +n B ) ~ n B /n A b. Molalility: m B = (moles solute)/(kg solvent) = n B /(n A M A ) Where M A is the mass in kg of one mole of solvent. There are FOUR Colligative Properties : 1. Vapor pressure lowering: p A = p A p A * = − x B p A * 2. Boiling point elevation: T b = T b T b * = K b m B * M R ( T ) 2 K b = A b H vap 3. Freezing point depression: T f = T f T f * = − K f m B * M A R ( T f ) 2 K f = H f 4. Osmotic pressure: π = RTc ~ where c ~ = n B is concentration of solute V
5.60 Spring 2008 Lecture #23 page 2 Let’s go through these one at a time: 1. Vapor pressure lowering: This is just Raoult’s Law.

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