Properties of Solutions

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Properties of Solutions
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Classification of Matter
Solutions are homogeneous mixtures
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Solute
A solute is the dissolved substance in a solution.
Salt in salt water
Sugar in soda drinks
Carbon dioxide in soda drinks
Solvent
A solvent is the dissolving medium in a solution.
Water in salt water
Water in soda
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Calculations of Solution Concentration
Mass percent - the ratio of mass (in grams) of
solute to mass (in grams) of solution, expressed
as a percent
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Calculations of Solution Concentration
Mass/volume (m/v) - the ratio of mass (in
grams) of solute to volume of solution (in mL),
expressed as a percent
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Calculations of Solution Concentration
Volume/volume (v/v) - the ratio of volume (in
mL) of solute to volume of solution (in mL),
expressed as a percent
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Calculations of Solution Concentration
Mole fraction the ratio of moles of solute to
total moles of solution
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Calculations of Solution Concentration
Molarity (M) - the ratio of moles of solute to
liters of solution
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Calculations of Solution Concentration
Normality (N) moles of equilvalents/Liter of
solution
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Calculations of Solution Concentration
Molality (m) moles of solute per kilogram of
solvent
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Heat of Solution
The Heat of Solution is the amount of heat energy
absorbed (endothermic) or released (exothermic)
when a specific amount of solute dissolves in a
solvent.
Substance Heat of Solution (kJ/mol)
NaOH -44.51
NH4NO3 25.69
KNO3 34.89
HCl -74.84
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Steps in Solution Formation
?H1 Expanding the solute
Separating the solute into individual components
?H2 Expanding the solvent
Overcoming intermolecular forces of the solvent
molecules
?H3 Interaction of solute and solvent to
form the solution
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Enthalpy Changes in Solution
The enthalpy change of the overall process
depends on ?H for each of these steps.
Start
End
Start
End
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Why do endothermic processes sometimes occur
spontaneously?
Some processes, like the dissolution of NH4NO3
in water, are spontaneous at room temperature
even though heat is absorbed, not released.
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Enthalpy Is Only Part of the Picture

• Entropy is a measure of
• Dispersal of energy in the system.
• Number of microstates (arrangements) in the
  system.
• b. has greater entropy, ? is the favored state

(more on this in chap 19)
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Like Dissolves Like
Nonpolar solutes dissolve best in nonpolar
solvents
Fats Benzene
Steroids Hexane
Waxes Toluene
Polar and ionic solutes dissolve best in polar
solvents
Inorganic Salts Water
Sugars Small alcohols
Acetic acid
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Predicting Solution Formation
Solvent/ Solute ?H1 ?H2 ?H3 ?Hsoln Outcome
Polar/ Polar large large - large /-small Solution forms
Polar/ Nonpolar small large /- small large No solution forms
Nonpolar/ Nonpolar small small /- small /- small Solution forms
Nonpolar/ polar large small /- small large No solution forms
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Solubility Trends

• The solubility of MOST solids increases with
  temperature.
• The rate at which solids dissolve increases with
  increasing surface area of the solid.
• The solubility of gases decreases with increases
  in temperature.
• The solubility of gases increases with the
  pressure above the solution.

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Solubility Chart
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Gases in Solution

• In general, the solubility of gases in water
  increases with increasing mass.
• Why?
• Larger molecules have stronger dispersion forces.

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Gases in Solution

• The solubility of liquids and solids does not
  change appreciably with pressure.
• But, the solubility of a gas in a liquid is
  directly proportional to its pressure.

Increasing pressure above solution forces more
gas to dissolve.
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Temperature

• Higher temperature drives gases out of solution.
• Carbonated soft drinks are more bubbly if
  stored in the refrigerator.
• Warm lakes have less O2 dissolved in them than
  cool lakes.

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Henrys Law
The concentration of a dissolved gas in a
solution is directly proportional to the pressure
of the gas above the solution
Applies most accurately for dilute solutions of
gases that do not dissociate or react with the
solvent
Yes ? CO2, N2, O2
No ? HCl, HI
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Therefore
Solids tend to dissolve best when

• Heated
• Stirred
• Ground into small particles

Gases tend to dissolve best when

• The solution is cold
• Pressure is high

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Saturation of Solutions

• A solution that contains the maximum amount of
  solute that may be dissolved under existing
  conditions is saturated.
• A solution that contains less solute than a
  saturated solution under existing conditions is
  unsaturated.
• A solution that contains more dissolved solute
  than a saturated solution under the same
  conditions is supersaturated.

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Degree of saturation

• Supersaturated
• Solvent holds more solute than is normally
  possible at that temperature.
• These solutions are unstable crystallization can
  often be stimulated by adding a seed crystal or
  scratching the side of the flask.

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Colligative Properties

• Colligative properties depend only on the number
  of solute particles present, not on the identity
  of the solute particles.
• Among colligative properties are
• Vapor pressure lowering
• Boiling point elevation
• Melting point depression
• Osmotic pressure

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Vapor Pressure
As solute molecules are added to a solution, the
solvent becomes less volatile (decreased vapor
pressure). Solute-solvent interactions
contribute to this effect.
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Raoults Law
The presence of a nonvolatile solute lowers the
vapor pressure of the solvent.
Psolution Observed Vapor pressure of
the solution
?solvent Mole fraction of the solvent
P0solvent Vapor pressure of the pure solvent
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What is the vapor pressure in mmHg of a solution
that contains 155 grams of glucose dissolved in
250. mL of water at 25.00 C? The vapor pressure
of pure water at 25.00C is 23.78 mmHg.





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Liquid-liquid solutions in which both components
are volatile
Modified Raoult's Law
P0 is the vapor pressure of the pure solvent PA
and PB are the partial pressures
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At a given temperature, you have a mixture of
benzene (vapor pressure of pure benzene 745
torr) and toluene (vapor pressure of pure toluene
290. torr). The mole fraction of benzene in
the solution is 0.450. Assuming both substances
are volatile, calculate the vapor pressure of the
solution.





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Colligative Properties of Electrolytes
Because these properties depend on the number of
particles dissolved, solutions of electrolytes
(which dissociate in solution) show greater
changes than those of nonelectrolytes. e.g. NaCl
dissociates to form 2 ion particles its limiting
vant Hoff factor is 2.
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The vant Hoff Factor, i
Electrolytes may have two, three or more times
the effect on boiling point, freezing point, and
osmotic pressure, depending on its dissociation.
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Dissociation Equations and the Determination of i
i 2
NaCl(s) ?
Na(aq) Cl-(aq)
i 2
AgNO3(s) ?
Ag(aq) NO3-(aq)
i 3
MgCl2(s) ?
Mg2(aq) 2 Cl-(aq)
i 3
Na2SO4(s) ?
2 Na(aq) SO42-(aq)
AlCl3(s) ?
Al3(aq) 3 Cl-(aq)
i 4
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vant Hoff Factor
One mole of NaCl in water does not really give
rise to two moles of ions.
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vant Hoff Factor
Some Na and Cl- reassociate as hydrated ion
pairs, so the true concentration of particles is
somewhat less than two times the concentration of
NaCl.
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The vant Hoff Factor

• Reassociation is more likely at higher
  concentration.
• Therefore, the number of particles present is
  concentration dependent.

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Boiling Point Elevation and Freezing Point
Depression
Solute-solvent interactions also cause solutions
to have higher boiling points and lower freezing
points than the pure solvent.
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Boiling Point Elevation and Freezing Point
Depression
In both equations, ?T does not depend on what
the solute is, but only on how many particles are
dissolved.
?Tb Kb ? i? m ?Tf Kf ? i? m
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Boiling Point Elevation
Each mole of solute particles raises the boiling
point of 1 kilogram of water by 0.51 degrees
Celsius.
Kb 0.51 ?C ? kilogram/mol
m molality of the solution
i vant Hoff factor
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Boiling Point Elevation
The change in boiling point is proportional to
the molality of the solution ?Tb Kb ?i?
m where Kb is the molal boiling point elevation
constant, a property of the solvent.
?Tb is added to the normal boiling point of the
solvent.
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If I add 45 grams of sodium chloride to 500.
grams of water, what will the boiling point be of
the resulting solution? Kb(H2O) 0.51 0C/m





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Freezing Point Depression
Each mole of solute particles lowers the freezing
point of 1 kilogram of water by 1.86 degrees
Celsius.
Kf 1.86 ?C ? kilogram/mol
m molality of the solution
i vant Hoff factor
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Freezing Point Depression

• The change in freezing point can be found
  similarly
• ?Tf Kf ?i? m
• Here Kf is the molal freezing point depression
  constant of the solvent.

?Tf is subtracted from the normal freezing point
of the solvent.
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If I add 92.0 grams of sodium chloride to 500.
grams of water, what will the freezing point be
of the resulting solution? Kf(H2O) 1.86 0C/m





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Freezing Point Depression and Boiling Point
Elevation Constants, ?C/m
Solvent Kf Kb
Acetic acid 3.90 3.07
Benzene 5.12 2.53
Nitrobenzene 8.1 5.24
Phenol 7.27 3.56
Water 1.86 0.512
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Osmotic Pressure
The minimum pressure that stops the osmosis is
equal to the osmotic pressure of the solution
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Osmotic Pressure

• The pressure required to stop osmosis, known as
  osmotic pressure, ?, is

where M is the molarity of the solution
If the osmotic pressure is the same on both sides
of a membrane (i.e., the concentrations are the
same), the solutions are isotonic.
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What is the osmotic pressure (at 85.0C) of
seawater? It contains approximately 27.0 grams of
NaCl per liter with an observed vant Hoff factor
of 1.89.





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SUMMARY OF EQUATIONS
i
i
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Molar Mass from Colligative Properties
We can use the effects of a colligative property
such as osmotic pressure to determine the molar
mass of a compound.
K
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Suspensions and Colloids
Suspensions and colloids are NOT solutions.
Suspensions The particles are so large that they
settle out of the solvent if not constantly
stirred.
Colloids The particles intermediate in size
between those of a suspension and those of a
solution.
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Types of Colloids
Examples Dispersing Medium Dispersed Substance Colloid Type
Fog, aerosol sprays Gas Liquid Aerosol
Smoke, airborn germs Gas Solid Aerosol
Whipped cream, soap suds Liquid Gas Foam
Milk, mayonnaise Liquid Liquid Emulsion
Paint, clays, gelatin Liquid Solid Sol
Marshmallow, Styrofoam Solid Gas Solid Foam
Butter, cheese Solid Liquid Solid Emulsion
Ruby glass Solid Solid Solid sol