110c-lecture18

# 110c-lecture18 - Problem Set#4 Due Wed May 12th Midterm 2...

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Problem Set #4 Due Wed May 12th. Midterm 2 Wed May 19 th . Material covered through end of Fri May 14 th . Chapters 23, 24, 25, and 26. Practice Midterm2 coming soon.

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Solutions of Electrolytes NaCl(s) + H 2 O(l) NaCl(aq) or CaCl 2 (s) + H 2 O(l) CaCl 2 (aq) + = m m dm m 0 2 2 , 2 1 1 ln φ γ * 1 1 2 ln 5 . 55 P P m = Electrolytes are non-ideal solutes. Vapor pressure for sucrose vs sodium chloride in water: CaCl 2 NaCl Sucrose Molality ln γ molality 000 0.2 0.09 0.16 0.4 0.18 0.32 0.6 0.27 0.47 0.8 0.37 0.63 1 0.46 0.79 1.4 0.67 1.12 1.8 0.89 1.46 Slope 0.49 0.81 Molality P 1 * - NaCl Sucrose NaCl(s) + H 2 O(l) Na + (aq) + Cl - (aq) Higher slope caused by: 1. Dissociation into two ions. (Colligative properties) 2. Solute-solute interactions (charge-charge) 3. Electrolytes violate Henry Law. ) ( ) 1 ( ) 1 ( * 1 1 * 1 2 * 1 1 2 1 2 2 2 2 P P P P P x x a = = = = sucrose P P 1 * 1 NaCl P P 1 * 1 2 , 2 2 m a m = Ideal in dilute soln (a 2 m 2 ) Non-ideal even in dilute soln (a 2 m 2 ) Henry’s Law
Experimental Mean Activity Coefficient ( γ ± ) ± γ determined by measuring osmotic coefficient ( φ ) (sec 25-2 pp1015-1019): 5 . 55 ln 2 1 φ ν m a = , where * 1 1 1 P P a = * 1 1 2 ln 5 . 55 P P m = Note, the extra factor of ν appears in the expression for φ . ( ν =1 for sucrose, ν =2 for NaCl and ν =3 for CaCl 2 ; ν = ν + + ν - ) Experimentally measure φ to calculate mean ionic activity coefficient: + = ± m dm m 0 / 1 2 / 1 2 1 1 ln eqn 25.42 p1029 ± ln is then used to calculate the mean ionic activity as ± ± ± = = m a a v 2 ν = # of dissolved species ( ν + + ν - )

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Debye-Huckel Limiting Law γ ± = 0.96 0.001 M γ ± = 0.89 0.01 M γ ± = 0.69 0.1 M What is ln γ ± for 0.001, 0.01 and 0.1 molar NaCl(aq)? 037 . 0 ) 001 . 0 ( 173 . 1 173 . 1 ln 2 / 1 = = = + ± c I z z 1173 . 0 ) 01 . 0 ( 173 . 1 173 . 1 ln 2 / 1 = = = + ± c I z z 37 . 0 ) 1 . 0 ( 173 . 1 173 . 1 ln 2 / 1 = = = + ± c I z z r q q RT ε πε κ µ 0 8 ln + ± ± = = 25-50 pm m 304 2 = T k q q B r 0 8 ln + ± = 2 / 1 173 . 1 ln m I z z c = + ± and deviates significantly from limiting law at m > 0.25 molal m vs ± ln
Chapter 25 Review 1. Non-electrolyte solutions (Henry’s Law). + = + = m m s molal m RT a RT , 2 2 2 , 2 2 ln 2 1 ln ln γ µ o o a 2,m m 2 and γ 2,m 1 as m 2 0 (m 2 < 10 -3 M) * 1 1 2 ln 5 . 55 P P m = φ and + = m m dm m 0 2 2 , 2 1 1 ln Partial pressure of water (P 1 ) from sucrose solution used to find m 2 .

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110c-lecture18 - Problem Set#4 Due Wed May 12th Midterm 2...

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