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Review

Review. To join clicker to class today: Turn on the Clicker (the red LED comes on). Push “Join” button followed by “20” followed by the “Send” button (switches to flashing green LED if successful). Conversion between molarity and (mass solute)/(mass sol'n).

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Review

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  1. Review • To join clicker to class today: • Turn on the Clicker (the red LED comes on). • Push “Join” button followed by “20” followed by the “Send” button (switches to flashing green LED if successful). • Conversion between molarity and (mass solute)/(mass sol'n). • Water solutions of electrolytes conduct electricity. • Water solutions of non-electrolytes (molecular compounds) do not conduct electricity. • Colligative properties. • Example 1: Osmosis • Osmotic pressure ∏= MRT • Calculated pressure of 0.50 M sugar relative to DI water. ∏=(0.50 mol/L)(0.08206 atm•L•mol-1•K-1)(298 K) = 12. atm (1.2 x 101 atm)

  2. Boiling Point Elevation • Solute in a liquid lowers the rate at which molecules can enter the gas phase by sticking to solvent molecules or blocking access to surface, raising the boiling point. • The observed effect is again proportional to the solute particle concentration: ∆Tb = imKb. • i = the van’t Hoff factor • m = molality = (moles solute/kg solvent) • Kb = the boiling point elevation constant which depends on the identity of the solvent.

  3. Molality (m) • Molality = (moles solute)/(kg of solvent) NOTE DENOMINATOR IS NOT kg of solution! • Ex. 0.50 moles NaCl in 2.0 kg water. m = 0.50 moles NaCl/(2.0 kg solvent) = 0.25 m NaCl in water. • Compare with molarity (M). What is the molality of 2.00 M NaCl in water ? • assume solution density =1.03 g/mL=>1.03 kg sol’n/L sol’n, • MM(NaCl) = 58.443 g/mole

  4. Boiling Point Elevation Calculation. • Relation: ∆Tb = imKb • Ex. How much higher is Bp of 0.500 m NaCl than pure water? • Kb(H2O) = 0.5121 K or ˚C/m • Assume i = ?

  5. Freezing Point Depression • Same idea as Bp elevation except that solute gets in the way of solid formation and lowers the freezing point. • Observed relation is: ∆Tf = imKf • Note that Kf is usually reported as positive • ∆T comes out positive. • You must subtract a positive ∆T from the freezing point of the pure solvent since the freezing point drops. • Other types of calculations you could do. • How much CaCl2 do you need in some amount of water to drop the freezing point 10 ˚C? • What morality CaCl2 sol'n is necessary? • Account for the van't Hoff factor.

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