Colligative Properties

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Colligative Properties
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• Does a solute change the properties of a solvent?

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

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Conductivity

• Pure water does not conduct electricity
• Some aqueous solutions do
• To conduct a solution must contain ions

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• Conductivity depends on nature of the particle
• molecular (covalent) solutes DO NOT conduct
• ionic solutes DO conduct

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Colligative Properties

• Depend on number particles in solution not type
  particles

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Colligative Properties

• doesnt matter what kind of particles (ions or
  molecules)
• Concentration of particles does matter

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What are some colligative properties?

• Vapor Pressure
• decreases (?)
• Freezing Point
• decreases (?)
• Boiling Point
• increases (?)
• change in Osmotic Pressure

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• The higher the concentration of solute particles,
  the larger the effect on colligative properties

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Does it matter if solute is ionic or covalent?

• YES
• number of particles will vary
• effected by concentration
• greater concen greater effect
• lower concen lesser effect

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Ionic solutes

• dissolve dissociate into () (-) ions ?
  more particles per mole of solute

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• MgCl2(s) H2O(l) ? Mg2(aq) 2Cl(aq)
• 1 mole salt ??3 moles ions
• NaCl(s) H2O(l) ? Na1(aq) Cl-1(aq)
• 1 mole salt ? 2 moles ions

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Covalent solutes

• dissolve as molecules ? 1-to-1 relationship
  between moles of solute
  moles of dissolved particles

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Covalent substances

• C6H12O6(s) H2O(l) ? C6H12O6(aq)
• 1 mole sugar ? 1 mole molecules

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

• The freezing point ? the boiling point ?
• The freezing point ? the boiling point ?
• The freezing point ? the boiling point ?
• The freezing point ? the boiling point ?

Answer 2
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Which solution containing 1 mole of solute
dissolved in 1000 g of water has the lowest
freezing point?

1. C2H5OH(aq)
2. NaCl(aq)
3. KOH
4. CaCl2

Be careful! - do you want greatest or least
change? What if question asked which solution
has highest freezing point?
Answer 4
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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

Answer 1
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Which solute, when added to 1000 g of water, will
produce a solution with the highest boiling point?

• 1) 29 g of NaCl
• 2) 58 g of NaCl
• 3) 31 g of C2H6O2
• 4) 62 g of C2H6O2

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

• 1) 1 g of NaCl dissolved per 100 g of H2O
• 2) 1 g of NaCl dissolved per 1000 g of H2O
• 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____ 0.017 mol ? 0.034 mol of
ions 58.8 g/mol (NaCl) (Na1
and Cl-1)

• 2) 1 g of C12H22O11 dissolved per 100 g of H2O

__1 g___ 0.0043 mol of molecules
232 g mol
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Effect of Nonvolatile Solute

• Boiling Point Elevation (?)
• Freezing Point Depression (?)
• The more 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