Colligative Properties

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Colligative Properties
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How does the solute change the properties of the
solvent?

• Consider aqueous solutions.
• Solvent water.
• How do the properties of the solution compare to
  the properties of pure water?

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Conductivity

• Pure water does not conduct electricity.
• Some solutions do.
• In order to conduct a current, a solution must
  contain ions. (MCPs)
• Conductivity depends on the nature of the
  particle Molecular solutes do not conduct.

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Colligative Properties

• Depend on the number of particles in solution not
  on the type of particles.
• Doesnt matter if particles are ions or
  molecules.
• Concentration of particles Does matter.

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What are some colligative properties?

• Vapor Pressure Lowering
• Freezing Point Depression
• Boiling Point Elevation
• Osmotic Pressure
• The higher the concentration of solute particles,
  the larger the effect.

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Does it matter if the solute is ionic or covalent?

• The number of particles will vary.
• Ionic solutes will dissolve to produce positive
  and negative ions ? more particles per mole of
  solute.
• For covalent solutes, 1-to-1 relationship between
  moles of solute and moles of dissolved particles.

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C6H12O6

• Covalent
• Dissolves as molecules
• C6H12O6(s) ? C6H12O6(aq)
• 1 mole of sugar yields 1 mole of molecules

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NaCl

• Ionic
• Dissolves as ions
• NaCl(s) ? Na(aq) Cl-(aq)
• 1 mole of salt yields 2 moles of ions. Get more
  particles from salt than sugar.

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MgCl2

• Ionic
• Dissolves as ions
• MgCl2(s) ? Mg2(aq) 2Cl-(aq)
• 1 mole of salt yields 3 moles of ions

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As a solute is added to a solvent, what happens
to the freezing point the boiling point of the
solution?

• The freezing point decreases the boiling point
  decreases.
• The freezing point decreases the boiling point
  increases.
• The freezing point increases the boiling point
  decreases.
• 4) The freezing point increases the boiling
  point increases.

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Which solution containing 1 mole of solute
dissolved in 1000 g of water has the lowest
freezing point?

• C2H5OH(aq)
• NaCl(aq)
• KOH(aq)
• 4) CaCl2(aq)

Be careful! What if the question asked which
solution has the highest freezing point?
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Of the following solutions, the one that will
freeze at the lowest temperature contains 1 mole
of nonvolatile solute dissolved in

• 1) 250 g of solvent
• 2) 500 g of solvent
• 3) 750 g of solvent
• 4) 1000 g of solvent

Lowest fr. pt. means most concentrated.
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Which solute, when added to 1000 g of water, will
produce a solution with the highest boiling point?
? Most concentrated.

• 29 g of NaCl
• 58 g of NaCl
• 31 g of C2H6O2
• 4) 62 g of C2H6O2

About 1 mole of NaCl. 2 moles of ions.
About 1 mole of C2H6O2. 1 mole of molecules.
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Which solution will freeze at the lowest
temperature?

• 1 g of NaCl dissolved per 100 g of H2O
• 1 g of NaCl dissolved per 1000 g of H2O
• 3) 1 g of C12H22O11 dissolved per 100 g of H2O
• 4) 1 g of C12H22O11 dissolved per 1000 g of H2O

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Which solution will freeze at the lowest
temperature?

• 1) 1 g of NaCl dissolved per 100 g of H2O
• 1 g ? 58.8 g/mole 0.017 moles ? 0.034 moles of
  ions.
• 3) 1 g of C12H22O11 dissolved per 100 g of H2O
• 1 g ? 232 g/mole 0.0043 moles of molecules

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Effect of Nonvolatile Solute

• Boiling Point Elevation
• Freezing Point Depression
• The more concentrated the particles, the bigger
  the effect!

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Applications of colligative properties

• Salting roads in the winter time.
• Using salted ice to make ice cream.