Chemistry Section 142

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ChemistrySection 14-2

• COLLIGATIVE PROPERTIES

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What are colligative properties?

• Properties of a solvent that are changed by the
  presence of a solute.
• Depend ONLY on the amount of dissolved solute,
  NOT on the identity of the solute.

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for NON-Electrolytes

• Vapor Pressure Lowering CP 1
• What is VP??
• Given a liquid in a closed container, some
  molecules will escape from the surface into the
  gas phase and exert pressure on the container
  walls.

• So, WHY does it go down when a solute is added??
• Solute particles compete with solvent for room at
  the surface, SO fewer solvent particles are
  available to escape into the vapor phase.

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for NON-Electrolytes, cont.

• Because VP has been lowered, the solution remains
  liquid over a larger range of temps. (graph
  p436) Thus
• Freezing Point Depression CP 2
• Boiling Point Elevation CP 3

• Dtf kf . m
• Dtb kb . m
• Dt temp change
• k constant (table p438)
• m molality
• For water,
• kf 1.86 oC/m
• kb 0.51 oC/m

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Lets Practice!

• p440 2
• The freezing point of an aqueous solution of
  glucose, C6H12O6, is found to be 0.325 oC. What
  is the concentration of this solution?
• 0.175 m

• p441 2
• A solution contains 450.0 g of sucrose,
  C12H22O11, dissolved in 250. g of water. What is
  the boiling point elevation AND the new boiling
  point of this solution?
• 2.68 oC, 102.68 oC

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for NON-Electrolytes, cont.

• Osmotic Pressure CP 4
• What is OP?? (diagram p442)
• Lets assume we have a U-tube with semi-permeable
  membrane dividing pure solvent on one side from a
  solution (solvent _______ ) on the other.
• The semi-permeable membrane allows some particles
  (solvent) to pass through but not others
  (solute).
• Solvent particles move more easily from the pure
  side to the solution side osmosis because
  WHY??

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for NON-Electrolytes, cont.

• on solution side, solute particles interfere
  with solvent contacting the membrane.
• So, the pure solvent level ________ and the
  solution level ________ until enough PRESSURE
  builds up on the solution side to force solvent
  particles back through to pure side at an equal
  rate (equilibrium!)
• So, as more solute is added, what happens to the
  amount of pressure required to establish
  equilibrium?
• Osmotic Pressure INCREASES as amount of solute
  increases.

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What about ELECTROLYTES?

• Recall that electrolytes __________ in water.
• Ex) 1 mole of Ba(NO3)2 dissociates into ___
  moles of ions.
• Therefore, molality (units ?? per ??) is
  TRIPLED!
• Since BP elev and FP dep are __________
  proportional to molality, they are also tripled!

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Lets Practice!

• p445 3
• The freezing point of an aqueous sodium chloride
  solution is 20.0 oC. What is the molality of
  this solution?
• 5.38 m NaCl

• Question
• How will you identify a substance like sodium
  chloride as an electrolyte if the problem doesnt
  specify??