# Chapter 17 Properties of Solutions

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```							                  Chapter 17
Properties of Solutions
17.1: Define solutions and the different
mathematical methods for describing solution
composition.
17.2: Provide a qualitative and pictorial
description of vapor pressure.
17.3: Discuss Raoult’s Law in a quantitative and
qualitative method for describing vapor
pressure.
17.4: Outline the role of colligative properties and
provide mathematical and qualitative
descriptions for colligative properties.

5/25/2012            Zumdahl Chapter 17           1
Solution Composition                  The solute and solvent can be any
combination of solid (s), liquid (l),
and gaseous (g) phases.

Dissolution:   There is an equal distribution of two unlike molecules

Solution: A homogeneous mixture
Solvent: The major component
5/25/2012
Solute: The minor component
Zumdahl Chapter 17                                         2
Mathematical Descriptions for Composition

Define: Mass Fraction, Mole Fraction, Molality
and Molarity
Mass percentage (weight percentage):
mass percentage of the component =
mass of solute/mass of solution *100

Mole fraction:

5/25/2012                      Zumdahl Chapter 17   3
Molality
m = mol of solute/ Kg of solvent

Molarity

Why do we use molality? Why is it important?
5/25/2012                         Zumdahl Chapter 17   4
What is Vapor Pressure?

5/25/2012
Important to Understand!
Zumdahl Chapter 17   5
Water at 20oC
Water at 30oC
Water at 50oC
Water at 80oC
Water at 100oC
Factors Affecting Solubility
Factors Affecting Solubility
1. Structure Effects
◦ Review chapter 4 – “Like Dissolves Like” concept
◦ Polar molecules, water soluble, hydrophilic
(water loving)
   E.g., Vitamins B and C; water-soluble
◦ Non-polar molecules, soluble in non-polar
molecules, hydrophobic (water fearing)
   E.g., Vitamins A, D, K and E; fat-soluble

5/25/2012                            Zumdahl Chapter 17     11
Factors Affecting Solubility
1. Structure Effects
2. Pressure Effects                       P = kH X
Henry’s Law (for dilute solutions)
The vapor pressure of a volatile solute (gas) in a
sufficiently dilute solution is proportional to the
mole fraction of the solute in the solution.
kH is Henry’s Law
constant
f (Temp, and the solute-
solvent interactions)

5/25/2012                       Zumdahl Chapter 17                12
Factors Affecting Solubility
1. Structure Effects
2. Pressure Effects
3. Temperature Effects for Aqueous Solutions
The solubility of several
solids as a function of
temperature.
The aqueous solubilities of a majority of
solids increase with increasing
temperature, some decrease with temp.
Endothermic – heat is added to a system
Exothermic – heat is removed from a
system
5/25/2012                  Zumdahl Chapter 17   13
Factors Affecting Solubility
1. Structure Effects
2. Pressure Effects
3. Temperature Effects for Aqueous Solutions
The solubility of several gases
in water as a function of
temperature at a constant
pressure of 1 atm of gas above
the solution.

Endothermic – heat is added to a system
Exothermic – heat is removed from a
system
5/25/2012                   Zumdahl Chapter 17   14
Mixtures

 Raoult’s Law
◦ Ideal Solutions
 One volatile (the solvent)
 One non-volatile (the solute)

P1 = X1 P°1

5/25/2012                              Zumdahl Chapter 17   15
Ideal Solutions and Raoult’s Law
 Consider a non-volatile solute (component 2)
in a solvent (component 1) which has a vapor
pressure
 X1 = mole fraction of solvent           Ideal

P1=X1 P°1
Raoult’s Law

Non-ideal solutions

Positive deviation is ____            Negative deviation is _

5/25/2012               Zumdahl Chapter 17                      16
Mixtures

 Raoult’s Law                       P1 = X1 P°1
◦ Ideal Solutions
 One volatile (the solvent)
 One non-volatile (the solute)

What happens if both are
volatile?
• One volatile (the solvent)
• One volatile (the solute)

P1 =X1 P°1
P2 =X2 P°2

5/25/2012
Ptot =P1 + P2         Zumdahl Chapter 17   17
Vapor pressure for a solution of two volatile liquids.

Positive deviation
= solute-solvent attractions < solvent-solvent attractions
For non-ideal Solutions

Negative deviation
5/25/2012                             Zumdahl Chapter 17                             18
= solute-solvent attractions > solvent-solvent attractions
Colligative Properties of Solutions
•        For some properties, the amount of
difference between a pure solvent and dilute
solution depend only on the number of
solute particles present and not on their
chemical identify.
•        Called Colligative Properties
•        Examples
–   Vapor Pressure             One’s we will observe

–   Boiling Point
–   Melting Point
–   Osmotic Pressure

5/25/2012                      Zumdahl Chapter 17                  19
Elevation of Boiling Point
– The boiling point of a solution of a non-volatile
solute in a volatile solvent always exceeds the
boiling point of a pure solvent
Lowering of Vapor Pressure
– Vapor Pressure of a solvent above a dilute
solution is always less than the vapor pressure
above the pure solvent.
Boiling
•       liquid in equilibrium with                                GAS
gas
Boiling Point
•       Vapor press = external
pressure                                          SOLID   Melting    LIQUID
Normal boiling point
Freezing
•       Vap press. = 1 atm
5/25/2012                                Zumdahl Chapter 17                           20
Elevation of Boiling                ΔT  K m
b solute
Point
and
ΔT is the boiling point elevation
Lowering of Vapor
Pressure                            Kb is molal boiling - point elevation constant
m              molality
is the molarlity of the solute in solution
solute

Phase diagrams for pure water (red
lines)
and for an aqueous solution
containing a nonvolatile
5/25/2012solution (blue lines).   Zumdahl Chapter 17                           21
ΔT  K m
b solute
ΔT  K m
f solute
Should be
negative
5/25/2012      Zumdahl Chapter 17                22
EXAMPLE
What is the minimum mass of ethylene glycol (C2H6O2) that
must be dissolved in 14.5 kg of water to prevent the solution
from freezing at -10oC?            Kf = 1.86 oC/m (water)

5/25/2012                 Zumdahl Chapter 17                    23
EXAMPLE
The boiling point of ethanol (C2H5OH) is 78.5oC. What is the boiling
point of a solution of 3.4 g of vanillin (Molar Mass = 152 g/mol) in
50.0 g of ethanol (Kb ethanol = 1.22oC/m).

Vanillin = solute                 moles of vanillin = 3.4g / 152(g/mol) = .022 mol
Ethanol = solvent
molality = .022mol / .050kg = .44m

delta [T] = .44m * 1.22 = .5

Tf = Ti + delta [T]

=79oC

5/25/2012                            Zumdahl Chapter 17                          24
Osmotic Pressure
Fourth Colligative Property
• Important for transport of
molecules across cell membranes,
called semipermeable
membranes
PV = nRT
• Osmotic Pressure = Π = g d h
Π = c RT
ΠV = n RT
ΠV = i n RT

c = Molarity or
moles/L or n/V
5/25/2012         Zumdahl Chapter 17                 25
Osmotic Pressure

The normal flow of solvent into the solution (osmosis) can be
prevented by applying an external pressure to the solution.

Osmotic Pressure useful for
 Determining the Molar Mass of
protein and other macromolecules
 small concentrations cause
large osmotic pressures
 Can prevent transfer of all solute
particles
 Dialysis at the wall of most
plant and animal cells

5/25/2012                         Zumdahl Chapter 17                              26
Dialysis: Representation of the
functioning of an artificial kidney
A cellophane (polymeric)
tube acts as the semi-
permeable membrane
 Purifies blood by
washing impurities
(solutes) into the
dialyzing solution.

5/25/2012            Zumdahl Chapter 17                         27
A dilute aqueous solution of a non-dissociating compound
contains 1.19 g of the compound per liter of solution and
has an osmotic pressure of 0.0288 atm at a temperature
of 37°C. Compute the molar mass of the compound.

5/25/2012              Zumdahl Chapter 17            28
Van’t Hoff Correction Factor
Colligative Properties of Electrolyte Solutions
Elevation of Boiling Point
ΔTb = m Kb
Where m = molality
(Molality is moles of solute per kilogram of solvent)
The Effect of Dissociation
ΔTb = i m Kb
i = the number of particles released into the
solution per formula unit of solute
e.g., NaCl dissociates into i = 2
e.g., Na2SO4 dissociates into i = 3                                also
(2 Na+ + 1 SO4-2)                                     Depression of
Freezing Point
e.g., acetic acid (a weak acid and weak
electrolyte) does not dissociate i = 1                    ΔTf = − m Kf
5/25/2012                          Zumdahl Chapter 17
ΔTf = − i m Kf
29

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