Mini-Unit: Colligative Properties

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Mini-Unit Colligative Properties
Day 2 - Notes

• Calculations with Colligative Properties

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After today you will be able to

• Explain what a colligative property is
• Calculate b.p. elevation and f.p. depression

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• Recall, colligative properties are properties of
  a solution which are affected by the number of
  dissolved particles in the solvent.
• The chemical makeup of the solute does not
  matter. The effect is the same. Sugar has the
  same affect as salt on a solvent.

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Affected Colligative Properties

• Three colligative properties that are affected
  are
• Vapor pressure (lowering)
• Boiling point (elevation)
• Freezing point (depression)
• \
• .There are others, but these are the ones we
  will talk about in this class.

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Boiling Point Elevation (?TB)

• Recall, boiling point is the point at which V.P.
  atmospheric pressure.
• i depends on the number of particles the solute
  breaks into
• KB a constant that depends on the solvent
• For H2O,
• m molality of the solution (mol/kg)

?TB (i)(KB)(m)
KB 0.512C/m
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Vant Hoff Factor (i)

• To obtain the i-value
• When dissolved, ionic compounds will break apart
  into separate ions.
• Examples NaCl ? i
• MgCl2 ? i
• Ca(SO4) ? i
• Ba3(PO4)2 ? i

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Na1 Cl-1
Mg2 2Cl-1
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Ca2 (SO4)-2
2
3Ba2 2(PO4)-3
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Example

• a) What is the change in the boiling point of a
  0.335m solution of KBr in H2O? b) What is the new
  boiling point?
• ?TB
• i
• m
• KB

?
?TB(i)(KB)(m)
KBr ? K1 Br-1
?TB(2)(0.512C/m)(0.335m)
0.335m
a) ?TB0.343C
0.512C/m
Water boils at 100C so
0.343C
100C


b) 100.343C
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Freezing Point Depression (?TF)

• Recall, freezing point is the temperature at
  which a liquid ? solid
• i depends on the number of particles
• KB a constant
• For H2O,
• m molality

?TF (i)(KF)(m)
KF 1.86C/m
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Example

• a) What is the change in the freezing point of a
  2.5m solution of BaF2 in H2O? b) What is the new
  freezing point?
• ?TF
• i
• KF
• m

?TF(i)(KF)(m)
?
?TF (3) (1.86C/m)(2.5m)
BaF2? Ba2 2F-1
a) ?TF -14C
1.86C/m
2.5m
Water freezes at 0C so
14C
0C
-

b) -14C
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Questions?Complete WS2