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(B) Adding a nonvolatile solute to a solvent decreases

Phase diagram. P. pure solvent. solid. solution. liquid. Note that FP and BP . 1 atm. gas. T. (B) Adding a nonvolatile solute to a solvent decreases the solution’s freezing point (FP) and increases its boiling point (BP) . (not water’s). NFP. NBP.

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(B) Adding a nonvolatile solute to a solvent decreases

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  1. Phase diagram P pure solvent solid solution liquid Note that FP and BP . 1 atm gas T (B) Adding a nonvolatile solute to a solvent decreases the solution’s freezing point (FP) and increases its boiling point (BP). (not water’s) NFP NBP

  2. The freezing point depression and boiling point elevation are given by: DTx = Kx m i DTx = FP depression or BP elevation Kx = Kf (molal FP depression constant) or Kb (molal BP elevation constant) -- they depend on the solvent -- for water: Kf= 1.86oC/m Kb = 0.52oC/m m = molality of solute

  3. i = van’t Hoff factor (accounts for # of particles in solution) In aq. soln., assume that… 1 -- i = __ for nonelectrolytes 2 -- i = __ for KBr, NaCl, etc. Jacobus Henricus van’t Hoff (1852 – 1911) 3 -- i = __ for CaCl2, etc. In reality, the van’t Hoff factor isn’t always an integer. Use the guidelines unless given information to the contrary.

  4. Kf = 1.86oC/m Kb = 0.52oC/m 0.690 m Find the FP and BP of a soln. containing 360. g barium chloride and 2.50 kg of water. DTx = Kx m i 360 g BaCl2 = 1.725 mol BaCl2 i = 3 DTf = Kf m i = 1.86(0.690)(3) = 3.85oC FP = –3.85oC DTb = Kb m i = 0.52(0.690)(3) = 1.08oC BP = 101.08oC

  5. 3.1oC 40.0 1 (organic = nonelec.) mol unknown Solve for m = 0.0775 kg camphor 1 mol unk. = 0.186 g unk. X g unk. Camphor, C10H16O, has an NFP of 179.8oC and a Kf of 40.0oC/m. When 0.186 g of a nonelectrolytic substance is dissolved in 22.01 g of camphor, the mixture’s new freezing point is 176.7oC. Find the unknown’s molar mass. DTf = Kf m i (Multiply by 0.02201 kg camphor to get…) 1.71 x 10–3 mol unk. X = 109 g/mol

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