Transcript Chapter 18

Chapter 18
Solutions
I. Solutions
A. Characteristics of solutions
1.
2.
3.
4.
5.
Homogeneous mixture
Contains a solute and solvent
Can be a gas, liquid or solid
Soluble - able to dissolve
Insoluble - not able to dissolve
6. Miscible - 2 liquids able to mix
7. Immiscible - 2 liquids not able to
mix
B. Dissolving mechanisms
1. Solute-solute attraction is broken
up, requiring energy
2. Solvent-solvent attraction is
broken up, requiring energy
3. Solute-solvent attraction is
formed, releasing energy
C. Solvation
1. When solvent particles surround
solute particles to form a solution
2. Depends on polarity and bonding
D. Factors that affect rate of dissolving
1. Agitating the mixture
2. Increase surface area
3. Increase temperature
4. Heat of solution (exothermic or
endothermic reactions)
E. Solubility
1. Ability to dissolve
2. Saturated solutions - maximum
amount of solute that can
dissolve
3. Unsaturated solutions - less
solute than a saturated solution
4. Supersaturated solution - has
more solute than a saturated
solution at the same
temperature
a. Formed at higher temp
b. Cooled slowly
5. As temperature is increased
solids dissolve faster
6. Lower temp. gases dissolve
better
7. Pressure affects gases not solids
F. Henry’s Law
1. At a given temp. the solubility (s) of a
gas in a liquid is directly proportional
to the pressure (P) of the gas above
the liquid
2. S1
P1
=
S2
P2
units = g/L
3. Ex.
If 0.24 g of a gas dissolves in 1.0 L
of water at 1.5 atm pressure, how
much gas will dissolve if the
pressure is raised to 6.0 atm? T
remains constant.
II.
Concentration of Solutions
A. Molarity
1. Concentration: amount of
solute that is dissolved in a
given quantity of solvent
a. Dilute solution: contains
a small amount of solute,
large amounts of solvent
b. concentrated solution:
Contains a small amount of
solvent and a large amount
of solute
2. Molarity (M)
a. Way to express
concentration
b. Number of moles of solute in
liters of solution
c.
. Example:
What is the molarity of an
aqueous solution that contains
14.2 g NaCl dissolved in 2364 mL
of the solution?
B. Making dilutions
1. Moles of solute before dilution = moles of solute
after
dilution
2. M1V1 = M2V2
3. Example:
How many milliters of a stock
solution of 2.00 M MgSO4 would
you need to prepare 100.0 mL of
0.400 M MgSO4?
C. Percent Solutions
1.
a. Ex.
What is the percent by mass of
sodium carbonate in a water
solution containing 0.497 g NaCO3
in 58.3 g of solution?
b. The % mass of MgCl2 in a water
solution is 1.47%. How many
g of solute are dissolved in
each 500.00 g of solution?
2.
a. Ex.
What is the %v/v of ethanol
(C2H6O) in the final solution
when 85 mL of ethanol is
diluted to a volume of 250
mL with water?
III. Colligative Properites
A. Definition: Properties that depend
only on the number of particels
dissolved in a given mass of
solvent
B. Vapor pressure
1. A vapor that is dynamic
equilibrium with its liquid in a
closed system
2. Dynamic equilibrium: when the
forward and the reverse
reactions are equal
3. If it contians nonvolatile solutes
(does not dissociate)
4. If it contains ionic compounds it
dissociates completely.
C. Boiling point – the temperature
difference between a solutions
boiling point and a pure solvent’s
boiling point
D. ΔTb (boiling point elevation) – directly
proportional to molality
E. molality
1. moles of solute per kg of solution
2. m = moles of solute
kg of solution
F . For nonelectrolyte(does’t dissociate)
1. ΔTb = Kbm
Kb – molal boiling
point elevation
constant
units  oC/m
m- molality
2. 1 m
or .512 oC
.512 oC
1m
for when water is the solvent
3. What is the boiling point elevation
when 31.5 g of methanol (C10H20O)
is dissolved in 258 g of acetic acid?
Kb for acetic acid is 2.93 oC/m.
4. How many grams of styrene glycol
(C8H10O2) must be dissolved in 98.7g
of bezene to raise the boiling point by
8.57 oC?Kb for benzene is 2.67oC/m.
G. Freezing point depression (ΔTf)
1. The temperature difference
between the freezing point of a
solution and the freezing point of
its pure solvent.
2. For nonelectrolytes
ΔTf = Kfm
3. When water is the solvent Kf = 1.86 oC/m
4. Calculate the freezing point of a solution
containing 5.70 g of sugar(C12H22O11) in
50.0 g of water.
5. If 13.4g of the medication
scopolamine, C17H21NO4, is
dissolved in 50.3 g of water, how
much will the freezing point be
lowered?
IV. Molar fraction and Electrolytes
A. Mole fraction
1. The ratio of the moles of solute
in solution to the total number
of moles of both solvent and
solute
2. Mole expressions
a. XA =
nA
nA + n B
(for solute)
b. XB = nB
nA + n B
(for solvent)
3. Ex. Compute the mole fraction of
each component in a solution of
1.25 mole of ethylene glycol
(EG) and 4.00 mole water.
B. Boiling point elevations
∆Tb = (# particles)(Kb)(m)
C. Freezing point depression
∆Tf = (# particles)(Kf)(m)
D. Molar mass
1. boiling point elevation
m = ∆Tb
Kb
2. freezing point depression
m = ∆Tf
Kf