LECTURE10 - Anomolous colligative properties of...

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1 Anomolous colligative properties of electrolytes Δ T f = -K f × m = -1.86°C m -1 × 0.0100 m = -0.0186°C 0.0100 m NaCl (aq) ? BUT Freezing point depression for NaCl is -0.0361°C. • van’t Hoff: Δ T f = -i K f m i = = = 1.98 measured Δ T f Δ T b = i K b m expected Δ T f 0.0361°C 0.0186°C • Arrhenius: Ions form when electrolytes dissolve in solution. Non-electrolyte : 1 mol solute 1 mol molecules, so i=1 Electrolytes: e.g. NaCl (strong electrolyte): Expect 2 mol ions / mol solute (i=2) To cook spaghetti more quickly, enough table salt (NaCl) is to be added to 2kg of water to raise the boiling point to 108°C. Can this be done? K b = 0.512 K kg/mol; Molar mass(NaCl) = 58.45 g/mol; Solubility (NaCl at 100C) ~ 45 g / 100g water Example 13F – Electrolyte B.P. Elevation
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2 Do not always get complete dissociation of ions: Strong electrolyte , e.g. MgCl 2 , i=3 Weak electrolyte , e.g. acetic acid: i is between 1 and 2 CH 3 COOH( aq )H + ( aq ) + CH 3 COO ( aq ) 2 HO   99% 1% Also, Debye and Hückel – Ions in solution do not behave independently: each ion is surrounded by those of opposite charge.
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This note was uploaded on 07/09/2011 for the course CHE 2B 2B taught by Professor Tobyallen during the Spring '11 term at UC Davis.

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LECTURE10 - Anomolous colligative properties of...

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