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III. Colligative Properties
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(p. 471 - 475)
Unit 11 - Solutions
(Ch. 15)
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Salting The Roads
Unit 11: Solutions
Lesson 11.3: Colligative Properties
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A. Definition
• Colligative Property
– property that depends on the concentration of solute particles, not their identity
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B. Types
• Lowering Vapor Pressure
• Freezing Point Depression (tf)
– f.p. of a solution is lower than f.p. of the pure solvent
• Boiling Point Elevation (tb)
– b.p. of a solution is higher than b.p. of the pure solvent
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A Strange Thing
Solutions have higher boiling points and lower freezing points than pure samples of their
solvents.
This has nothing to do with the type of solute.
It only has to do with the amount.
These properties (that have to do with the amount of a solute, not the type) are called
“Colligative Properties”.
It explains:
– Why antifreeze works.
– Why we salt the roads before snowstorms.
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Why should this be?
Remember the hydration shell?
When we dissolve something in water, the water molecules surround the solute particles and
weakly “bond” to them (IMAF’s).
To get water to evaporate from a solution, more energy has to be put in to break the
attractions between water and solute particles.
So, solutions have higher and higher boiling points as we dissolve more and more solute in
them.
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Boiling Point Elevation
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But what about freezing?
When water freezes, it has to form a crystal lattice.
This is harder to do if there are solute particles in the way.
So, even more energy has to be removed to get the crystal lattice to form.
The more solute in the solution, the lower is the freezing point.
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• Applications
– salting icy roads
– making ice cream
– antifreeze
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So, even more energy has to be removed to get the crystal lattice to form.
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The more solute in the solution, the lower is the freezing point.
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• Applications
– salting icy roads
– making ice cream
– antifreeze
• cars (-64°C to 136°C)
• fish & insects
–
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Dissolving Electrolytes
When we dissolve an electrolyte (e.g. a salt), we get more particles in solution:
NaCl (s)  Na+ (aq) + Cl- (aq)
For every mole of NaCl we dissolve, we get 2 moles of dissolved ions.
This breaking apart is called a “dissociation”.
It is a physical change.
How many moles of ions will we get if we dissolve 1 mole of CaBr 2? What about Al(NO3)2?
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Dissolving Non-Electrolytes
Non-electrolytes do not dissociate in water:
C6H12O6(s)  C6H12O6(aq)
We only get as many particles in solution as we start with when we dissolve electrolytes.
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Which of the following solutions will boil at the highest temperature?
a) 100 g NaCl in 1000 g of water
b) 100 g NaCl in 500 g water
c) 100 g NaCl in 250 g of water
d) 100 g NaCl in 125 g of water
e)
e)
e)
Why?.....
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Which of the following solutions will boil at the lowest temperature?
a) 100 g NaCl in 1000 g of water
b) 100 g NaCl in 500 g water
c) 100 g NaCl in 250 g of water
d) 100 g NaCl in 125 g of water
e)
e)
e)
Why?......
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Which of the following solutions will freeze at the lowest temperature?
a) 100 g NaCl in 1000 g of water
b) 100 g NaCl in 500 g water
c) 100 g NaCl in 250 g of water
2
d) 100 g NaCl in 125 g of water
e)
e)
e)
Why?......
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Which of the following solutions will freeze at the lowest temperature?
a) 100 g NaCl in 1000 g of water
b) 100 g NaCl in 500 g water
c) 100 g NaCl in 250 g of water
d) 100 g NaCl in 125 g of water
e)
e)
e)
Why?
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Which of the following solutions will freeze at the highest temperature?
a) 100 g NaCl in 1000 g of water
b) 100 g NaCl in 500 g water
c) 100 g NaCl in 250 g of water
d) 100 g NaCl in 125 g of water
e)
e)
e)
Why?
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Which of the following solutions will boil at the highest temperature?
a) 100g C6H12O6 in 500 g of water
b) 100g KBr in 500g of water
c) 100g AlCl3 in 500g of water
d) 100g MgF2 in 500g of water
c)
c)
c)
Why?
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Which of the following solutions will boil at the lowest temperature?
a) 100g C6H12O6 in 500g of water
b) 100g KBr in 500g of water
c) 100g MgF2 in 500g of water
d) 100g AlCl3 in 500 g of water
c)
c)
c)
Why?
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Which of the following solutions will freeze at the lowest temperature?
a) 100g C6H12O6 in 500g of water
b) 100g KBr in 500g of water
c) 100g MgF2 in 500g of water
d) 100g AlCl3 in 500 g of water
c)
c)
c)
Why?
3
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Which of the following solutions will freeze at the lowest temperature?
a) 100g C6H12O6 in 500g of water
b) 100g KBr in 500g of water
c) 100g MgF2 in 500g of water
d) 100g AlCl3 in 500 g of water
c)
c)
c)
Why?
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Which of the following solutions will freeze at the highest temperature?
a) 100g C6H12O6 in 500g of water
b) 100g KBr in 500g of water
c) 100g MgF2 in 500g of water
d) 100g AlCl3 in 500g of water
c)
c)
c)
Why?
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C. Calculations
t: change in temperature (°C)
k: constant based on the solvent (°C·kg/mol)
m: molality (m)
n:
# of particles
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Calculating Freezing Point of Solutions
For every mole of particles dissolved in 1kg (aka 1L) of H2O, the freezing point of the
solution decreases by 1.86℃
1.Figure out how many moles of particles are in the solution (the “molality” of the solution)
2.Multiply #1 by 1.86 ℃
3.Subtract #2 from the freezing point of H2O (aka 0℃)
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Calculating Boiling Point of Solutions
For every mole of particles dissolved in 1kg (aka 1L) of H2O, the boiling point of the solution
increases by 0.52℃
1.Figure out how many moles of particles are in the solution (the “molality” of the solution)
2.Multiply #1 by 0.52 ℃
3.Add #2 to the boiling point of H2O (aka 100℃)
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C. Calculations
• # of Particles
– Nonelectrolytes (covalent)
• remain intact when dissolved
• 1 particle
– Electrolytes (ionic)
• dissociate into ions when dissolved
• 2 or more particles
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C. Calculations
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C. Calculations
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C. Calculations
• Find the freezing point of a saturated solution of NaCl containing 28 g NaCl in 100. mL
water.
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• dissociate into ions when dissolved
• 2 or more particles
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C. Calculations
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C. Calculations
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C. Calculations
• Find the freezing point of a saturated solution of NaCl containing 28 g NaCl in 100. mL
water.
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