Properties of Solutions

1
Properties of Solutions

• Chapter 11

2
Composition of Solutions

• Solutions homogeneous mixtures,
  any state of matter

Solid
Gas
Liquid
Gas
Liquid
Solid
3
How to Calculate Concentration
Mole Fraction
Molarity
Molality
Mass Percent
Normality
(mol solutecation charge) L solution
4
Energies of Solutions

• Like Dissolves Like
• Polar solvents dissolve polar solutes
• Non-Polar solvents dissolve non-polar solutes
• WHY?
• ENTHALPY OF SOLUTION

• 3 part process
• Solute components separate (endo)
• Overcome IMFs in solvent (endo)
• Solvent and Solute interact (exo)

5
Enthalpy of Solution

• DHsoln DH1 DH2 DH3
• Heat of solution can either be or
• DH is expected to be small for non-polar
  molecules except for when they are very large
  molecules.

DHsoln is large and This rarely occurs
DHsoln is large and - This usually occurs
DHsoln is close to zero Exceptions,
because Spontaneity does not Depend solely on
enthalpy
6
Try Me!

• Decide whether liquid hexane (C6H14) or liquid
  methanol (CH3OH) is more appropriate solvent for
  substances, grease (C20H42) and potassium iodide
  (KI).

7
Factors Affecting Solubility

• Structure---Polarity
• Non polar are hydrophobic
• Polar are hydrophillic
• Some are a combo with different ends.
• Vitamin C and scurvy
• Pressure---works only on gases
• C kP (Henrys Law)
• gas a Pvapor
• Temperature---not always a direct relationship
• Gas in water as T increases, solubility
  decreases
• See solubility curves for individual compound
  reactions to temperature.

8
Colligative Properties

• A property of a solution that depends only on the
  number, and not on the identity of the solute
  particles present.
• Vapor Pressure
• Boiling Point Elevation
• Freezing Point Depression
• Osmotic Pressure

9
Vapor Pressure

• Non-volatile Solute added to solvent
• Reduces the tendency of the solvent molecules to
  escape (evaporate)
• Equilibrium pressure of the solvent is higher
  than equilibrium pressure of solution. More of
  the solvent (pure) has to evaporate to be at
  equilibrium than is the case in a beaker full of
  solution.

10
François Raoult and his Law

• Vapor pressure of a solution is directly
  proportional to the mole fraction of the solvent
  present.

Psoln XsolventPosolvent
Just what I was thinking!!
Me Too!
I agree!
11
Try Me!

• Calculate the expected vapor pressure at 25oC for
  a solution prepared by dissolving 85.58 g of
  table sugar (sucrose molar mass 342.3 g/mol) in
  320.5 cm3 of water. The vapor pressure of water
  is 23.76 torr at this temperature.

12
Answer

• 23.431 torr

13
More solute, lower vapor pressure

• Predict the vapor pressure of a solution prepared
  by mixing 35.0 g solid Na2SO4 with 175 g water
  at 25oC. The vapor pressure of water is still
  23.76 torr.

14
Volatile solutes

• Raoults Law is modified to account for the vapor
  pressure of both solute.
• Ptotal Pa Pb XaPao XbPbo
• Always some degree of deviation from idealmore
  similar the solute is to the solvent the more
  ideal it behaves.

15
Boiling Point Elevation

• When a solute is added, vapor pressure is
  lowered.
• Must be heated to higher temperature to give
  reach normal boiling point.
• Boiling point is higher than usual.
• DT Kbmsolute

16
Freezing Point Depression

• When a solute is added, the vapor pressure is
  lowered.
• Water in the solution has a lower vapor pressure
  than that of pure ice.
• Ice will not form.
• Freezing point has been lowered.
• DT Kfmsolute

17
Try Me!

• A chemist is trying to identify a human hormone,
  which controls metabolism by determining its
  molar mass. A sample weighing 0.546 g was
  dissolved in 15.0 g benzene, and the freezing
  point depression was determined was determined to
  be 0.240oC. Calculate the molar mass of the
  hormone.

18
Osmotic Pressure

• Solvent, but not solute can pass through a
  semi-permeable membrane via osmosis.
• When pure solvent and solution volumes are
  stabilized (the system is at equilibrium) there
  is a greater pressure on the solution side.
• The difference in pressure is the Osmotic
  Pressure.

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P MRT

• Used to determine osmotic pressure.
• P is osmotic pressure
• M is molarity of solution
• R is the gas law constant (0.0821 Latm/Kmol)
• T is the temperature in Kelvin

20
Colligative Properties of Electrolyte Solutions

• The type of solute DOES matter sometimes
• Only in ability to dissociate
• Compare 0.1 m glucose to 0.1m NaCl

21
Vant Hoff Factor

• Vant Hoff Factor expresses the relationship
  between moles of solute and solution
• This is a multiplier number that describes the
  ratio of elevation or depression.

i moles particles moles solute
DT iKbm
DT iKfm
P iMRT
22
Ion Pairing

• Vant Hoff factor is fallible.
• Ion Pairing at a certain instant, some ions may
  not be dissociated, and thus count as a single
  particle.
• Still use equations, but keep in mind that there
  can be some deviation.

23
Colloids

• Suspension of tiny particles in a medium caused
  by electrostatic repulsion.
• Tyndall Effect Scattering of light effect by
  suspended particles.