Ways of Expressing Concentration

1
Ways of Expressing Concentration

• Mole Fraction, Molarity, and Molality
• We define
• Converting between molarity (M) and molality (m)
  requires density.

2
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?

3
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)
4
Ways of Expressing Concentration
Mole Fraction, Molarity, and Molality
5
Saturated Solutions and Solubility

• Mole Fraction, Molarity, and Molality
• Dissolve solute solvent ? solution.
• Crystallization solution ? solute solvent.
• Saturation crystallization and dissolution are
  in equilibrium.
• Solubility amount of solute required to form a
  saturated solution.
• Supersaturated a solution formed when more
  solute is dissolved than in a saturated solution.

6
Factors Affecting Solubility

• Solute-Solvent Interactions
• Polar liquids tend to dissolve in polar solvents.
• Miscible liquids mix in any proportions.
• Immiscible liquids do not mix.
• Intermolecular forces are important water and
  ethanol are miscible because the broken hydrogen
  bonds in both pure liquids are re-established in
  the mixture.
• The number of carbon atoms in a chain affect
  solubility the more C atoms the less soluble in
  water.

7
Factors Affecting Solubility
Solute-Solvent Interactions
8
Factors Affecting Solubility

• Solute-Solvent Interactions
• The number of -OH groups within a molecule
  increases solubility in water.

9
Factors Affecting Solubility

• Solute-Solvent Interactions
• Generalization like dissolves like.
• The more polar bonds in the molecule, the better
  it dissolves in a polar solvent.
• The less polar the molecule the less it dissolves
  in a polar solvent and the better is dissolves in
  a non-polar solvent.
• Network solids do not dissolve because the strong
  intermolecular forces in the solid are not
  re-established in any solution.

10
Factors Affecting Solubility

• Pressure Effects
• Solubility of a gas in a liquid is a function of
  the pressure of the gas.
• The higher the pressure, the more molecules of
  gas are close to the solvent and the greater the
  chance of a gas molecule striking the surface and
  entering the solution.
• Therefore, the higher the pressure, the greater
  the solubility.
• The lower the pressure, the fewer molecules of
  gas are close to the solvent and the lower the
  solubility.

11
Factors Affecting Solubility
Pressure Effects
12
Factors Affecting Solubility

• Pressure Effects
• Henrys Law
• Cg is the solubility of gas, Pg the partial
  pressure, k Henrys law constant.
• Carbonated beverages are bottled under gt
  1 atm. As the bottle is opened,
  decreases and the solubility of CO2 decreases.
  Therefore, bubbles of CO2 escape from solution.

13
Factors Affecting Solubility

• Temperature Effects
• Experience tells us that sugar dissolves better
  in warm water than cold.
• As temperature increases, solubility of solids
  generally increases.
• Sometimes, solubility decreases as temperature
  increases (e.g. Ce2(SO4)3).

14
Factors Affecting Solubility
Temperature Effects
15
Factors Affecting Solubility

• Temperature Effects
• Experience tells us that carbonated beverages go
  flat as they get warm.

16
Factors Affecting Solubility

• Temperature Effects
• Experience tells us that carbonated beverages go
  flat as they get warm.
• Gases are less soluble at higher temperatures.
• Thermal pollution if lakes get too warm, CO2 and
  O2 become less soluble and are not available for
  plants or animals.

17
Colligative Properties

• Colligative properties depend on quantity of
  solute molecules. (E.g. freezing point
  depression and boiling point elevation.)
• Lowering the Vapor Pressure
• Non-volatile solvents reduce the ability of the
  surface solvent molecules to escape the liquid.
• Therefore, vapor pressure is lowered.
• The amount of vapor pressure lowering depends on
  the amount of solute.

18
Colligative Properties
Lowering the Vapor Pressure
19
Colligative Properties

• Raoults Law
• Raoults Law PA is the vapor pressure with
  solute, PA? is the vapor pressure without
  solvent, and ?A is the mole fraction of A, then
• Recall Daltons Law