Solutions and Problems
Remember– if you’re not part of
the solution, you’re part of the
precipitate.
• Like dissolves Like.
• Generally, non-polar solvents dissolve
non-polar solutes
• Generally, polar solvents dissolve polar
and ionic solutes
• Generally, metallic solvents dissolve
metallic solutes
Solution Problem
• If you need to dissolve KBr, what solvent
would you choose?
• If you need to dissolve iodine, what
solvent would you choose?
• If you need to dissolve silver, what solvent
would you choose?
• Can you defend your choice by IM forces?
• But: not all ionic compounds dissolve in
water
• Compounds with acetate, hydrogen,
nitrate, ammonium, group 1 metals,
chlorate and perchlorate ions are soluble
• Insoluble compounds might contain Ag+,
Pb+2, Hg2+2, carbonate, phosphate, sulfide
or hydroxide
Solution Problem
•
Which compounds will not dissolve well in
water?
a. K2CO3
b. AgCl
c. Pb(NO3)2
d. FeCl3
e. NH4OH
f. Ba(OH)2
g. SrCO3
h. Ag2S
i. PbI2
j. CuCl2
k. Fe(OH)3
l. BaSO4
Why not?
• Their lattice energies are too high for the
attraction of water molecules to separate
the ions.
• Stronger or shorter ionic bonds, maybe.
Water, the universal solvent
• Almost anything dissolves, at least a little,
in water.
• Consider the entropy—dissolving even a
little increases entropy.
But….
• Most solids dissolve more in warm water
(the exceptions all have a positive heat of
solution)
• Gasses dissolve less in warm water (the
gas wants to become a gas at higher
temperatures?)
• Solubility of gasses increases (directly!) at
higher pressures.
• Henry’s law
s1
P1
=
s2
P2
Solution Problem
• If .16 mg H2 dissolves in 100 g H2O at
20oC when the pressure is 1.0 atm, what
is the molality of this solution?
– a. How much H2 will dissolve at 3.5 atm (at
20oC)?
– b. How much H2 will dissolve at 50oC (when
the pressure is 1.0 atm)?
• Solubility of gasses is quite low, though,
unless a reaction occurs.
NH3 (g) + H2O NH4+ + OHHCl (g)  H+ + ClCO2 (g) + H2O H2CO3
• (any non-metallic oxide in water makes an
acid)
What’s a solution like?
• Usually, it depends what you dissolve.
– Color (can you name a blue, green, and
colorless solution?)
– Density (can you name solutions more and
less dense than water?)
– pH (can you name solutions with acidic and
basic pH’s?)
– Viscosity (can you name solution more and
less viscous than water?)
Colligative properties
• Some of the properties of solutions don’t
depend on the type of solute—just how
much is dissolved.
• These are the colligative properties
• The colligative properties depend upon the
mole fraction of the solution.
• Since the fraction of the solvent is usually
much greater than the solute, we often
convert moles of solvent to kilograms, and
use the molality—using a conversion for
that solvent
The colligative properties are…
• Boiling point elevation
– The boiling point of a solution of a nonvolatile solute
is greater than the pure solvent
• Freezing point depression
– The freezing point of a solution is less than the pure
solvent
• Vapor pressure depression
– The vapor pressure of a solution of a nonvolatile
solute is less than the pure solvent
• Osmotic pressure (increase)
– Solutes increase the osmotic pressure of a solution
The van’t Hoff factor
• When using a mole fraction, it’s the mole
fraction of particles that matters
• For molecular solutes, i=1.
• For weak acids, i=(just over) 1, depending
upon Ka, the strength of the acid.
• For NaCl, i=(almost) 2; in fact, i=1.9 This
value represents the (almost complete)
extent of dissociation.
Solution Problem
• What (approximately) is the van’t Hoff
factor for…
Na2SO4
H2SO4
CH3OH
FeCl3
KI
HF
H2S
HBr
Na2O
Boiling point elevation
DTb=iKbm
Where
DTb is the change in the boiling point
(compared to the BP of the solvent)
i is the van’t Hoff factor of the solute
Kb is the boiling point constant for that
solvent and
m is the molality of the solution
What is i?
What is i?
• I don’t know, who are you? Who taught
you grammar?
Solution Problem
• What is the boiling point of an aqueous .85
m NaCl solution?
Solution Problem
• If a 1.0 m aqueous solution of an ionic
compound has a boiling point of 101.34oC,
what is i?
Solution Problem
• If a 72 g a non-ionic, nonvolatile
compound dissolved in 100 g water has a
boiling point of 101.02oC, what is its
formula mass?
Freezing point depression
DTf=iKfm
Where
DTf is the change in the freezing point
(compared to the MP of the solvent)
i is the van’t Hoff factor of the solute
Kf is the freezing point constant for that
solvent and
m is the molality of the solution
Solution Problem
• If a you dissolve 1.1 g of a molecular
solute in 8.7 g H20 and measure the
freezing point at -2.5oC, what is the
formula mass of the solute?
Solution Problem
• If you dissolve 28 g CCl4 in 121 g
benzene, at what temperature will it
freeze?
Solution Problem
(for those who can’t look it up)
• If you dissolve 28 g CCl4 in 121 g
benzene, at what temperature will it
freeze?
• FM CCl4 =154 g/mol
• MP Benzene= 5.50C
Vapor pressure depression
• Even simpler.
VPsolution=
Where
x
solventis
x
solventx
VPsolvent
the mole fraction of the
solvent, in terms of particles in the solution
as long as the solute can’t vaporize
Solution Problem
• What is the vapor pressure of an aqueous
2.1 m NaCl solution at 100.0oC?
Now it gets less simple…
• …but more useful.
• What if the solute and the solvent can
vaporize?
• If you have two miscible liquids, each one
has its own vapor pressure—but it doesn’t
matter which one you call the solvent
Raoult’s Law
VPsolution=
x x VP + x x VP
1
1
2
2
You just add up the contribution from each
component.
Solutions that follow Raoult’s law are called
ideal solutions —it’s a good estimate for
many solutions.
Solution Problem
• What is the vapor pressure of a mixture of
28 g methanol and 35 g ethanol at 20oC?
Solution Problem
(for those who can’t look it up)
• What is the vapor pressure of a mixture of
28 g methanol and 35 g ethanol at 20oC?
• VP pure methanol = 98 mmHg
• VP pure ethanol=42 mmHg
Special Bonus Problem Solution!
• The mole fractions of the vapor are the
vapor pressure fractions of the gas above
the solution—
• Xvapor =VP1/ (VP1 + VP2)
= .27 for ethanol
= .73 for methanol
Way different than the solution. This is the
key to distillation
Osmotic pressure
• Osmosis is the diffusion of water across a
semipermeable membrane—permeable to
water, but not (all) solutes
• Osmotic pressure is usually useful in
biological systems.
• The rule is, “Water follows salt”
• How did we get more water on the right?
• Obviously, it is under pressure. Osmotic
pressure, in this case.
Osmotic pressure moves water
P, the osmotic pressure
P=iMRT
• Where
P is the osmotic pressure
i is the van’t Hoff factor
M is the molarity
R is the ideal gas constant and
T is the absolute temperature
P, the osmotic pressure is a
pressure!
P=iMRT
• Where i has no units,
M is in mol L-1
R is in L atm mol-1K-1
T is in K
P is the osmotic pressure in
(mol L-1) (L atm mol-1K-1 )(K )
=atmospheres
P, the osmotic pressure is a
pressure!
or…P=iMRT
• Where i has no units,
M is in mol L-1
R is in L kPa mol-1K-1
T is in K
P is the osmotic pressure in
(mol L-1) (L kPa mol-1K-1 )(K )
=kilopascals