Solutions

1
Solutions

• Chapter 12

2
Saturated Solutions and Solubility

• If the solvent cannot solvate any more molecules
  the solution is saturated.
• The solubility of a solid in a solvent is
  typically given in g/100 ml.

3
Factors Affecting Solubility

• Intermolecular Forces
• Hydrogen bonding
• Like dissolves Like
• Is this correct?

H
H
Na
Cl-
O
4
Factors Affecting Solubility

• Intermolecular Forces
• Hydrogen bonding
• Like dissolves Like

H
H
O
H
H
Cl-
Na
O
H
O
H
5
Factors Affecting Solubility

• Intermolecular Forces
• Hydrogen bonding
• Like dissolves Like

H
H
H
O
O
H
H
H
Cl-
Na
O
H
O
H
6
Factors Affecting Solubility

• Intermolecular Forces
• Hydrogen bonding
• Like dissolves Like

H
H
O
H
H
H
Cl-
Na
O
O
H
H
O
H
Solvation Separates the Ions.
7
Factors Affecting Solubility

• Temperature - increased kinetic energy of the
  solvent induces more molecular interactions with
  the solute. But Does it?
• Not for gases where more kinetic energy in the
  solvent decreases the number of gas molecules
  trapped in the liquid.

8
Factors Affecting Solubility

• Pressure - Henrys Law
• S kH P
• S solubility of a gas
• kH Henrys Law constant
• P partial pressure of gas
• Increasing the pressure of a gas above a liquid
  increases its solubility

9
Ways of Expressing Concentration

• Molarity
• moles of solute/liter of solution
• Normality
• equivalents of solute/liter of solution
• Molality
• moles of solute/kg of solvent

10
Solution Concentration

• Fructose, C6H12O6, is a sugar occurring in honey
  and fruits. How much water should be added to
  1.75 g of fructose to give a 0.125 m solution?

11
Solution Concentration

• Convert mass of fructose to moles and then
  multiply by 1 kg H2O per 0.125 mol fructose (the
  reciprocal of molality).

1 mol fruct.
1 kg H2O
1.75 g fruct.
x
x
180.16 g fruct.
0.125 mol fruct.
0.077708 kg (77.7 g H2O)
12
Colligative Properties

• Many properties of solutions depend only on the
  number of molecules or atoms present.

13
Colligative PropertiesVapor Pressure

• Raoults Law
• Psoln ?solvent Psolvent

14
Deviations from Raoults Law

• Intermolecular forces between components in a
  dissolved solution cause deviations from the
  adjustment to vapor pressure.

Pvap A
Vapor Pressure
Pvap B
?A
15
A Problem

• 20 g of an unknown solid is dissolved in 125 g of
  water at 25oC. What is the molecular weight of
  the solid if the observed vapor pressure was
  21.72 torr and pure water usually has a vapor
  pressure of 23.76 torr at this temperature?

16
The Solution

• Ptot ?H O?PH O?
• ?H2O???Psoln /PH O 21.72/23.76 0.9141
• Or
• ?H2O???mol H2O/mol H2O mol unknown

2
2
2
17
The Solution

• Ptot ?H O?PH O?
• ?H2O???Psoln /PH O 21.72/23.76 0.9141
• Or
• ?H2O???mol H2O/mol H2O mol unknown

2
2
2
0.9141 125 g /(18 g/mol H2O)
125 g /(18 g/mol H2O) 20 g /( X g/mol)
18
The Solution

• Ptot ?H O?PH O?
• ?H2O???Psoln /PH O 21.72/23.76 0.9141
• Or
• ?H2O???mol H2O/mol H2O mol unknown

2
2
2
0.9141 125 g /(18 g/mol H2O)
125 g /(18 g/mol H2O) 20 g /( X g/mol)
X 30.87 g/mol
19
Colligative Properties

• Boiling Point Elevation
• ?????Tb Kbcm
• Freezing Point Depression
• ?Tf Kf cm

Where cm is the molal concentration of the
solution
20
Colligative PropertiesOsmotic Pressure

• When two solutions are separated by a
  semi-permeable membrane, solvent molecules will
  transfer through the membrane to equalize the
  concentration.

21
Colligative PropertiesOsmotic Pressure

• When two solutions are separated by a
  semi-permeable membrane, solvent molecules will
  transfer through the membrane to equalize the
  concentration - OSMOSIS.

Osmotic Pressure is the pressure necessary
to prevent osmosis.
? (n/V)RT MRT
22
Non-Dissolved Solutions

• Colloids are small particles that are not
  precipitated by the gravity or intermolecular
  interactions.
• Hydrophilic, water loving
• -OH, -NH2
• Hydrophobic, water fearing
• -CH3, aromatics, long chain aliphatics

23
Colligative Properties

• A 0.0182-g sample of an unknown substance is
  dissolved in 2.135 g of benzene. The molality of
  this solution, determined by freezing-point
  depression, is 0.0698. What is the molecular
  weight of the unknown substance?

24
Colligative Properties

• Find the moles of unknown solute from the
  definition of molality
• Molsolute m x kg solvent

0.0698 mol

x 0.002135 kg solvent
1 kg solvent
1.490 x 10-4 mol
25
Colligative Properties

• Find the moles of unknown solute from the
  definition of molality
• Molsolute m x kg solvent

0.0698 mol

x 0.002135 kg solvent
1 kg solvent
1.490 x 10-4 mol
0.0182 g
Molar mass
122.1 122 g/mol
1.490 x 10-4 mol
(molecular weight 122 g/mol)