Chapter 13 Sec. 2

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Chapter 13Sec. 2

• Solutions and Their Colligative Properties

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Solutions have them but pure substances do not
have them.

• Colligative Properties - Properties of solutions
  which depend on the number of solute particles in
  the solution and not the nature of the solute.

Three Colligative Properties
1) Vapor pressure lowering 2) Boiling point
elevation 3) Freezing point depression
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II. Units of Concentration
Molarity and mass are not useful in
colligative properties because the exact amount
of solvent is unknown.
Only molality concentration units reflect the
number of solute particles per solvent molecules
and are useful with colligative properties.
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III. Colligative Properties
Vapor pressure the pressure above a liquid
created by the liquid releasing gas molecules as
it evaporates. The pressure is directly related
to temperature.
A. Vapor Pressure Lowering
The vapor pressure of the solution is lowered
because the solute particles at the liquid/vapor
boundary block the solvent particle from jumping
into the vapor state.
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Dynamic Equilibrium

• When the rate of freezing is the same as the rate
  of melting, the amount of ice and the amount of
  water won't change on average (although there are
  short-term fluctuations at the surface of the
  ice). The ice and water are said to be in dynamic
  equilibrium with each other.
• The balance between freezing and melting
  processes can easily be upset. If the ice/water
  mixture is cooled, the molecules move slower. The
  slower-moving molecules are more easily captured
  by the ice, and freezing occurs at a greater rate
  than melting.
• Molecules in Motion Flash plug-in

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IV. Colligative Properties
B. Boiling Point Elevation
For nonvolatile, nonelectrolyte solvents the
change in boiling point (DTbp) is
Kbp boiling point elevation constant
DTbp Kbpmsolute
msolute molality of solute
The Boiling point Curve is shifted to the right.
Raising the boiling Pt.
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IV. Colligative Properties
C. Freezing Point Depression
For nonvolatile, nonelectrolyte solvents the
change in freezing point (DTfp) is
msolute molality of solute
The Freezing point curve shifts to the left.
Lowering the F.P. of the solvent.
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Practice Problems

• 1. Determine the freezing point of a solution of
  60.0 g of glucose, C6H12O6, dissolved in 80.0 g
  of water.
• m mol/kg
• m 60.0g / 180g/mol / 0.080kg 4.167m
• ?Tfp (-1.86C/m) (4.167 m)
• ans -7.75C

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Practice Problems

• 2. What is the boiling point of a solution of 645
  g of urea, CON2H4, dissolved in 980. g of water?
  .
• m mol/kg
• m 645g / 60g/mol / .980 kg 10.97m
• ?Tbp (0.512C/m) (10.97 m)
• ?Tbp 5.62C
• BP 100 5.62C 105.6C

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IV. Colligative Properties
D. Colligative Properties of Solutions
Containing Ions
The change in VP, BP or FP is greater than
expected for electrolyte (ionic salt) solutions.
Predicted BP elevation of an aqueous 0.100 m NaCl
solution
DTbp Kbp msolute
DTbp, calculated (0.512 C/m)(0.100 m) 0.0512
C
Actual BP elevation of an aqueous 0.100 m NaCl
solution
DTbp, measured 0.09470 C (Almost double the
DTbp calculated)
Colligative properties depend on the total
number of solute particles in solution. Ionic
compounds form ions in solution so the total
number of solute particles in solution is equal
to the total ions in solution.
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IV. Colligative Properties
D. Colligative Properties of Solutions
Containing Ions
NaCl(s) ? Na(aq) Cl-(aq)
0.100 m
0.100 m
0.100 m
0.200 m total
THE MULTIPLIER -vant Hoff factor (i) We will
assume 100 ionization
So for electrolytes or ionic solutions
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IV. Colligative Properties
D. Colligative Properties of Solutions
Containing Ions
Predicting vant Hoff factors
NaCl(s) ? Na(aq) Cl-(aq)
1 particle 1 particle 2 particles
ipredicted 2
Na2SO4(s) ? 2 Na(aq) SO42-(aq)
2 particles 1 particle 3 particles
ipredicted 3
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Practice Problems

• 3. What is the expected boiling point of a brine
  solution containing 30.00 g of KBr dissolved in
  100.00 g of water?
• m mol/kg KBr ? two ions , i 2
• m 30.00g / 119.0g/mol / .100 kg 2.52 m
• ?Tbp 2 (0.512C/m) (2.52 m) 2.6C
• B.P. of water 100.0C 2.6C
• ans 102.6C

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Practice Problems

• 4. What is the expected boiling point of a CaCl2
  solution containing 385 g of CaCl2 dissolved in
  1.230 kg of water?
• m mol/kg CaCl2 ? three ions , i 3
• m 385 g / 111.1 g/mol / 1.230 kg 2.82 m
• ?Tbp 3 (0.512C/m) (2.82 m) 4.3C
• B.P. of water 100.0C 4.3C
• ans 104.3C

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IV. Colligative Properties
E. Colligative Properties and Molar Mass
Determination
Colligative properties can be used to determine
the molar mass of a solute when it is dissolved
in a solvent of appreciable vapor pressure and a
known Kbp or Kfp.
Example Butylated hydroxyanisole (BHA) is used
as an antioxidant in margarine and other fats and
oils it prevents oxidation and prolongs the
shelf life of food. What is the molar mass of BHA
if 0.640 g of the compound, dissolved in 25.0 g
of chloroform, produces a solution whose boiling
point is 62.22 C. (Chloroform BP 61.70 C, Kbp
3.63 C/m)
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Practice Problems

• 5. A solution of 0.827 g of an unknown
  non-electrolyte compound in 2.500 g of water has
  a freezing point of -10.18C. Calculate the molar
  mass of the compound.
• Molar mass g/mol solve for moles thru m
• -10.18 (-1.86C/m) (m) m 5.47
  mol/kg
• m 5.47 mol/kg mol/0.0025kg mol .01367
• Molar mass 0.827g /.01367 mol
• ans 60.5 g/mol