CH 11: Properties of Solutions
1.
2.
3.
4.
Describing Solutions – concentration units
Energetics of solution formation
Factors impacting solubility – P and T
Colligative Properties of solutions
•
•
•
•
BP elevation
FP depression
Osmotic pressure
Vapor Pressure
Terms
• Solution – homogeneous mixture
• Solvent – generally the larger component of
the solution
– Determines the physical state of the solution
• Solute – generally the smaller component of
the solution
– Solute is dispersed in the solvent
Solution Composition
• Concentrated solution – relatively large
amount of solute
• Dilute solution – relatively small amount of
solute
Solution Composition
• Unsaturated solution –solution with less than
the maximum amount of solute that will
normally dissolve at a given temperature
• Saturated solution - solution with maximum
amount of solute that will normally dissolve at
a given temperature
Solution Composition
• Super-saturated solution - solution with more
than the maximum amount of solute that will
normally dissolve at a given temperature
Concentration Units
Molarity (M) = moles solute/Liters solution
Molality (m) = moles solute/kg solvent
Mass % = Mass solute/mass solution x100%
Mole fraction (cA) = moles A/total moles
Normality
Normality (N) = # gram equivalent wt./L solution
– Gew = mass of an acid or base that produces 1
mole of H+ or OH -1
Normality = Molarity x (# H+ or OH -1 /mol)
– 1 M HCl = 1 N HCl
– 1 M H2SO4 = 2 N H2SO4
For acid base titrations:
(NAcid) (VAcid) = (NBase) (VBase)
Practice!
• Start by writing definitions for the
concentration units
M=
m=
Mass % =
Mole fraction =
Starting with Molarity
Solution:
– 3.75 M H2SO4 solution with a density of 1.23 g/mL
Calculate:
– Mass %
– Molality
– mole fraction of H2SO4
Starting with Masses
Solution:
– A solution is made by combining 66.0 grams of
acetone (C3H6 O) with 146.0 grams of water.
– Solution has a density of 0.926 g/mL
Calculate:
–
–
–
–
Molarity – need volume of solution
Mass %
Molality
Mole fraction of acetone
Starting with Mass %
Solution:
– 35.4 % H3PO4
– Density of 1.20 g/mL
Calculate:
– Molarity
– Molality
– Mole fraction of H3PO4
Starting with Molality
Solution:
– 2.50 m HCl solution
– Density of 1.15 g/mL
Calculate:
– Molarity – need _______
– Mass %
– Mole fraction of HCl
Solution Formation
Formation of a solution involves 3 steps
1. Separate the solute particles
•
expand the solute
2. Separate the solvent particles
•
Expand the solvent
3. Form the solution
–
Solute and solvent interact
Solution Formation
• Each step of solution formation involves
energy and has a DH.
DH1 = energy needed to separate the
solute
DH2 = energy needed to separate the
solvent
DH3 = energy released when solution
forms
Solution Formation
DHsolution = DH1 + DH2 + DH3
Solutions form when the DHsolution is a small
value – see page 504
Factors Impacting Solubility
• Structure – like dissolves like
– #44 on page 533
Factors Impacting Solubility
• Pressure
– Pressure has little impact on the solubility of
liquids and solids
– Pressure has a significant impact on the solubility
of gases in a liquid
• The higher the pressure of gaseous solute above a
liquid the higher the concentration of the gas in the
solution
Henry’s Law
• Henry’s Law: C = kP
C = Concentration of dissolved gas
k = solution specific constant
P = partial P of the solute gas above
the solution
• What does Henry’s Law mean?
– No calculations required.
Page 506
Temperature and Solubility
• Temperature has variable effects on the
amount of solid that will dissolve in an
aqueous solution!
– See figure 11.6 page 508
• Solutes will dissolve more rapidly at higher
temperatures
Temperature and Solubility
• The solubility of a gas in water decreases as
temperature increases.
– See figure 11.7 on page 508
– Thermal pollution – read the story on page 510
when you get a chance
Vapor Pressure of Solutions
• See Raoult’s Law on page 510
• Psolution = csolvent P0 solvent
Colligative Properties
• Colligative properties
– properties of a solution that depend upon the
amount of dissolved solute, not the identity of the
solute.
• Freezing point depression
• Boiling point elevation
• Osmotic Pressure
• Note: I will be weaving section 11.7 and the van’t Hoff
factor (i) into my consideration of these properties and
not consider it separately.
Colligative Properties
D FP = Kf m i
D BP = Kb m i
See page 517 for needed constants
1. Calculating the bp or fp of a solution
2. Calculating the molar mass of a solute from
fp or bp data
Osmotic Pressure
• Osmotic Pressure (P) is often used to
determine the molar mass of large biological
molecules
P = MRTi