17.3 Factors Affecting Solubility:

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17.3 Factors Affecting Solubility
Structure Effects

• Having a dipole moment allows molecule to
  dissolve in water
• Molecules with a good number of O-H, N-H,
• C-O, and CO bonds tend to be water soluble
• Molecules with few polar groups (mostly C-C and
  C-H bonds) tend to be insoluble in water, but
  soluble in non-polar solvents

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Octane
Tartaric Acid
Hydrophilic (soluble in water, polar organic
solvents)
Hydrophobic (soluble in oils, organic solvents)
Riboflavin
Cholesterol
Hydrophilic
Hydrophobic
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Some Polar Solvents

• Water
• Ethanol
• Acetone
• Acetonitrile

4
Some Non-polar Solvents

• Benzene
• Chloroform

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Factors Affecting Solubility Pressure Effects

• Very little effect on solubility of solids or
  liquids
• Large effect on solubility of a gas
• Higher gas pressure above liquid, more gas
  dissolved in liquid

Fig 17.4 pg 855
Start
Increase Pressure
New Equilibrium
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Henrys Law

• P kH ?

or P kH C
Partial Pressure of gaseous solute above solution
Mole fraction of gas in solution
units of L atm / mol
Molarity of gas in solution
Henrys Law Constant units of atm
Works best for dilute solutions of gases that
dont interact with solvent
Example from Email CO2 has a Henry's law
constant of 1.64x103 atm. What is the mole
fraction of CO2 dissolved in water when the
pressure of CO2 above the solution is 1.35atm?
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Factors Affecting Solubility Temperature Effects

• Solids, liquids dissolving in a solvent
• Solubility often increases
• with temperature,
• but not always
• Hard to predict

Fig. 17.5 pg 856
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Factors Affecting Solubility Temperature Effects

• Gases dissolving in a solvent
• Solubility decreases
• with temperature
• kH is described for a
• given temperature

Fig. 17.6 pg 858
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17.4 The Vapor Pressures of Solutions

• A nonvolatile solute lowers the vapor pressure of
  a solvent
• The solute decreases the number of solvent
  molecules at the surface
• This lowers the escaping tendency (and vapor
  pressure) of the solvent molecules

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Raoults Law

• Psoln Xsolvent Posolvent

Vapor Pressure of Solution
Vapor Pressure of Pure Solvent
Example from Email Vapor pressure of water is
23.8 torr at 25C. If you have a solution of
water and the nonvolatile solute glycerin at
25C, where the mole fraction of the glycerin is
0.23, what is the vapor pressure above that
solution?
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Another Example

• Poethanol 59.0 torr at 25C
• M.W. of ethanol 46.07 g/mol
• 20.0g of glucose was added to 60.0 g ethanol at
  25C.
• The vapor pressure of the solution is 54.4 torr.
• Molar mass of glucose?

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Raoults Law, Two Volatile Components

• Components A and B in an ideal binary solution
• Ptotal PA PB XA PoA XB PoB

Question from Email What is the composition of
an ethanol - propanol solution with a vapor
pressure of 125 torr at 45C? at 45C
Poethanol 173 torr and Popropanol 74 torr