Properties of Solutions

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Properties of Solutions

• Chapter 13 BLB 11th

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Expectations

• g ? mol (using molar mass)
• g ? mL (using density)
• Other conversions temp., pressure, etc.
• Solve for any variable in a formula.
• Distinguish between molecular and ionic
  compounds.
• Convert between different concentration units.

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13.1 The Solution Process

• Solution homogeneous mixture
• Solute present in smaller quantity
• Solvent present in larger quantity
• Intermolecular forces are rearranged when a
  solute and solvent are mixed.

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Making a Solution

• Solute molecules separate (endothermic)
• Solvent molecules separate (endothermic)
• Formation of solute-solvent interactions
  (exothermic)
• ?Hsoln total energy
• ?Hsoln enthalpy change for the formation of a
  solution exothermic usually favorable
  endothermic usually unfavorable

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Will a solution form?

• Solute-solvent interaction must be stronger or
  comparable to the separation of solute and
  solvent particles.
• Intermolecular forces play a key role.
• Entropy (disorder) is also a factor.
• Disorder is favorable. (2nd law of
  thermodynamics)
• Solution formation increases entropy.
• Dissolve vs. react (p. 533-4)

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Entropy in Solution Formation
As the ionic compound dissolves, it becomes more
disordered.
Ionic compound very ordered
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13.2 Saturated Solutions and Solubility

• Saturated solution solution is in equilibrium
  with undissolved solute.
• Solute solvent ? solution
• Unsaturated less solute than saturated
• Supersaturated more solute than saturated

dissolution
crystallization
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A Saturated Solution

• A dynamic equilibrium ions continually exchange
  between the solid and solution form.

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13.3 Factors Affecting Solubility

• Like dissolves like, i.e. same polarity.
• Polar solutes are soluble in polar solvents.
• Nonpolar solutes are soluble in nonpolar
  solvents.
• If two liquids miscible or immiscible
• Examples
• ? water alcohol, NaCl water, hexane pentane
• ? water hexane, NaCl benzene, oil water

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Fat- and Water-Soluble Vitamins
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13.3 Factors Affecting Solubility

• Pressure Effects (for gases in any solvent)
• Solubility increases as the partial pressure
  above the solution increases.
• Henrys Law Sg kPg
• Sg solubility of gas
• k Henrys Law constant conc./pressure units
• Pg partial pressure of gas above solution

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The partial pressure of O2 in your lungs varies
from 25 to 40 torr. What molarity of O2 can
dissolve in water at each pressure? The Henrys
Law constant for O2 is 6.02 x 10-5 M/torr.
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13.3 Factors Affecting Solubility

• Temperature Effects
• For solids Solubility ? as temperature ? -
  usually.
• If ?Hsoln gt 0 (endothermic)
• If ?Hsoln lt 0 (exothermic)
• For gases Solubility ? as temperature ? -
  always.
• Kinetic energy plays a primary role.
• Entropy is also a factor.

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Ioniccompounds
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Gases
(In liquids)
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13.4 Ways of Expressing Concentration

• Mass , volume , and ppm

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13.4 Ways of Expressing Concentration, cont.

• Mole fraction, molarity, and molality

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44. A solution contains 80.5 g ascorbic acid
(C6H8O6) in 210 g water and has a density of 1.22
g/mL at 55C.Calculate mass , X, m, and M.
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51. Commercial aqueous nitric acid has a density
of 1.42 g/mL and is 16 M. Calculate mass of
HNO3.
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43. A sulfuric acid solution containing 571.6 g
of H2SO4 per liter of solution has a density of
1.329 g/cm3. Calculate the mass , mole
fraction, molality, and molarity.
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Concentration Problems

• Practice!
• See Figure 13.19, p. 545, for conversion map.
• Several examples on pp. 544-6.

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13.5 Colligative Properties

• The addition of a solute to a pure solvent
• Lowers the vapor pressure
• Lowers the freezing point
• Raises the boiling point
• Causes movement through a semipermeable membrane
  (osmosis)
• Depends on the number of solute particles
  (moles), not the identity more particles the
  greater the effect
• Ionic compounds cause an even greater effect.

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1. Lowering the vapor pressure

• Addition of solute blocks the solvent from
  evaporation.
• More solute, less vapor, lower vapor pressure
• Raoults Law (for a nonvolatile solute)
• PA XAPA PA solvent v. p. over solution
• (PA lt PA) PA pure solvent v. p.
• XA mole fraction of solvent

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1. Lowering the vapor pressure, cont.

• When a volatile solute is added, both the solvent
  and solute contribute to the vapor pressure.
• Expanded Raoults Law
• Ptotal PA PB XAPA XBPB
• If a solution obeys Raoults Law, it is an ideal
  solution.
• Nonideal solutions have strong intermolecular
  interactions which lower the vapor pressure of
  the solution even further.

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Vapor Pressure Lowering
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62a. Calculate the vapor pressure above a
solution of 32.5 g C3H8O3 (glycerin-nonvolatile)
in 125 g water at 343 K. The vapor pressure of
water at 343 K is 233.7 torr.
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63. A solution is made from equal masses of water
and ethanol (C2H5OH). Calculate the vapor
pressure above the solution at 63.5C. The vapor
pressures of water and ethanol are 175 and 400.
torr, respectively, at 63.5C.
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2. Boiling point elevation3. Freezing point
depression

• Since a solution has a lower vapor pressure
• A higher temperature is needed to boil solution
• A lower temperature is needed to freeze solution.
• To calculate effect
• b.p. ? ?Tb Kbm solution - solvent
• f.p. ? ?Tf Kfm solvent - solution
• ?T difference between boiling or freezing
  points of the pure solvent and solution
• K boiling or freezing pt. dep. constant
  (specific to solvent)
• m molality

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69a. Calculate the freezing and boiling points of
a solution that is 0.40 m glucose in ethanol.For
ethanol f.p. -114.6C, b.p. 78.4C, Kf 1.99
C/m, Kb 1.22 C/m
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72. Calculate the molar mass of lauryl alcohol
when 5.00 g of lauryl alcohol is dissolved in
0.100 kg benzene (C6H6). The freezing point of
the solution is 4.1C. For benzene f.p. 5.5C,
Kf 5.12 C/m
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4. Osmosis

• Osmosis movement of solvent molecules through a
  semipermeable membrane from a region of lower
  solute concentration to a region of higher solute
  concentration
• Driving force to dilute the higher
  concentration
• Continues until
• Equilibrium is reached between two solutions, or
• External pressure prevents further movement.

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Osmosis in red blood cells
Hypertonic solution
Hypotonic solution
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4. Osmosis, cont.

• Osmotic pressure
• P M R T P osmotic pressure (atm)
• M molarity
• R 0.08206 Latom/molK
• T temperature (K)
• Good technique for measuring molar mass of large
  molecules like proteins

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4. Osmosis, cont.

• Applications
• Kidney dialysis
• Intercellular transport
• Reverse osmosis apply external pressure to
  reverse the flow of solvent molecules
• Water purification alternative to salt ion
  exchange
• Desalination purification of salt water

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78. A dilute aqueous solution of an organic
compound is formed by dissolving 2.35 g in water
to form 0.250 L of solution. The resulting
solution has an osmotic pressure of 0.605 atm at
25C. Calculate the molar mass of the compound.
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13.6 Colloids

• Colloid or colloidal dispersion
• Intermediate between a solution and a suspension
• Dispersing medium analogous to solvent
• Dispersing phase analogous to solute typically
  large molecules with high molar masses
• Does not settle
• Tyndall effect particles scatter light

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Tyndall Effect
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Types of Colloids
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Surfactants

• Change the surface properties so that two things
  that would not normally mix do
• Emulsifying agent
• Soap
• Detergent

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Hydrophobic water-fearing (nonpolar)Hydrophilic
water-loving (polar)
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Action of soap on oil