Solutions - WebAssign

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Chapter 13.
Solutions
Solutions are homogeneous mixtures of two
or more substances.
Each substance retains its chemical identity.
The particles are of atomic or molecular size.
Solutions
Solutions can be solids, liquids, or gases.
Solid solutions: Metal alloys
Liquid solutions: Numerous
Gaseous solutions: Air!
Is it a solution? Liquid and gaseous solutions
are transparent (not necessarily colorless).
Solutions
Definitions:
Solvent:
the major component, in which
others are dissolved.
Solute(s): minor component(s), active
ingredients in a solution.
Solution:
Solvent + Solute(s)
Solubility
Solubility is the maximum amount of solute
that will dissolve in a solvent.
What dissolves in what?
Like Dissolves Like
Polar substances dissolve in polar solvents.
Nonpolar substances dissolve in nonpolar
solvents.
Solubility
Water is a very polar solvent, and will dissolve
some, but not all, ionic compounds.
Ionic compounds are less likely to dissolve in
water if:
a. the ions have charges other than +/- 1.
b. The bonds are really polar covalent.
Rate of Solution Formation
Small particles of solute dissolve faster
than large particles.
Stirring makes the process go faster.
Getting the solvent hot makes the process
go faster.
How Much Solute Will Dissolve?
Increasing the temperature usually increases
the solubility of a compound.
How Much Solute Will Dissolve?
High pressure increases the solubility of
gases. (Henry’s Law)
There is usually an optimal temperature
for solubility of gases.
How Much Solute Will Dissolve?
When dissolved gases come out of solution,
their volume expands drastically!
This occurs when the pressure is released.
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How Much Solute Will Dissolve?
How Much Solute Will Dissolve?
A saturated solution contains the maximum
amount solute that can dissolve in the
solvent under specified conditions.
Undissolved solute may be present.
An unsaturated solution does not contain
the maximum amount of solute that can
dissolve in the solvent under specified
conditions.
How Much Solute Will Dissolve?
The concentration of a solution is a ratio
of the amount of solute per amount of
solvent, or per amount of solution.
Solute
Solution
Solute
Solvent
Concentration: Percent of Solute
Percent by mass (mass %, % w/w)
% by mass = Mass of Solute x 100%
Mass of Solution
Percent by volume (volume %, % v/v)
% by volume = Volume of Solute x 100%
Volume of Solution
Concentration: Percent of Solute
Problems:
What is the concentration of a solution containing 0.290 g of H2S in 75.00 g of water? (V-1)
How many grams of glucose are present in 75 g
of a solution that is 10.% (m/m) glucose?
How many grams of iodine (I2) must be added
to 25.0 g of alcohol to make a solution that is
5.00% by mass?
Concentration: Percent of Solute
Problems:
How many milliters of isopropanol (C3H8O) are
present in 500. mL of a 70.0% (v/v) solution
in water?
What volume of water must be added to 25.0
mL of H2SO4 to make 100.0 mL of 25.0%
(v/v) solution?
Concentration: Percent of Solute
When two different liquids are combined,
volumes are not additive.
Volumetric flasks are filled to the 50.0 mL mark
with ethanol and with water. The liquids are
combined in a 100 mL volumetric flask, and
the volume is 96.5 mL.
Concentration: Percent of Solute
Percent by mass-volume (% m/v)
% m/v =
Mass of Solute (g)
x 100%
Volume of Solution (mL)
Since units do not cancel, they must be
specified.
% m/v is convenient for preparing solutions of
solids in liquids, but not as versatile as other
percent concentrations.
Concentration: ppm and ppb
ppm = Parts per Million
ppb = Parts per Billion
(m/m), (v/v) or (m/v)
ppm (m/m) = Mass of Solute x 106
Mass of Solution
ppb (v/v) = Volume of Solute x 109
Volume of Solution
Concentration: mg/dL
mg/dL = milligrams per deciliter
1 mg = 0.001 g
1 dL = 0.1 L = 100 mL
1 mg/dL =
1 mg solute .
100 mL of solution
Frequent in medical reports; it's
unambiguous about units.
Concentration: Molarity
Molarity = Moles of Solute
Liters of Solution
Abbreviated M, or mol/L
[solute], molarity of solute, in equations
The concentration used by chemists. It allows
one to convert from moles to volume.
mol
L
L
mol
Concentration: Molarity
Volume x Molarity = Moles
How many moles of NaCl are present in 100.
mL of 0.500 M solution?
Concentration: Molarity
Problems:
What is the molarity of 1.00 L of solution that
contains 25.0 g of NaCl? (V-1)
What is the molarity of a solution if 2.0 g of glucose is dissolved to give 15 mL of solution?
How many moles of glucose are present in
3.0 mL of the solution?
How many milliters of solution would be
needed to obtain 0.0065 moles of glucose?
Concentration Conversions
Sometimes one needs to know the
concentration of a solution in units
other than those on the container.
Often, conversions can be made if one
knows the density of the solution.
Concentration Conversions
A saturated solution of NaOH is 50.% by
mass. Its density is 1.52 g/mL What is
the molarity of the solution?
A what is the mass percent of glucose in a
0.10 M solution? The density of the solution is 1.0 g/mL, and the molar mass of
glucose is 180. g/mol.
What is the molarity of pure water?
Dilution of Solutions
Dilution is the process of adding solvent to a
known volume of solution to reduce its
concentration.
It is often useful to keep a stock (concentrated)
solution in the laboratory.
It can be diluted to lower concentrations as
needed.
Dilution calculations are simple:
C1V1 = C2V2
Dilution of Solutions
If I take 15 mL of that CuSO4 solution I made,
and dilute it to 100. mL with water…
Can I tell by looking that I made a difference?
What is the concentration of the dilute solution,
in molarity? in mass percent? (V-2)
Dilution of Solutions
Problems:
The most concentrated form of HCl(aq) is
12.0 M. How much of this solution is required to prepare 500. mL of 3.0 M HCl?
What is the concentration of a solution
made by diluting 25.0 mL of 50.% (w/w)
NaOH to 200 mL?
How many milliters of 3.0 M HNO3 can be
prepared from 15.0 mL of 17 M HNO3?
Solution Stoichiometry
One can use molarities in theoretical yield type
calculations like molar masses. If all solutions have the same molarity, combining
volumes will match reaction coefficients!
2 AgNO3(aq) + K2CrO4(aq) 
Ag2CrO4(s) + 2 KNO3(aq)
If we start with 5.0 mL of 0.10 M AgNO3, what
volume of 0.10 M K2CrO4 will be required to
complete the reaction?
Solution Stoichiometry
If molarities don't match, a theoretical yield
calculation sorts things out:
2 AgNO3(aq) + K2CrO4(aq) 
Ag2CrO4(s) + 2 KNO3(aq)
If we start with 5.0 mL of 0.10 M AgNO3, what
volume of 0.50 M K2CrO4 will be required to
complete the reaction?
Solution Stoichiometry
If we're very careful with our measurements, we
can use a solution of known molarity to
determine the molarity of another solution.
AgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq)
When all the NaCl is used up, AgNO3 reacts
with K2CrO4 and there is a color change!
2 AgNO3(aq) + K2CrO4(aq) 
Ag2CrO4(s) + 2 KNO3(aq)
Solution Stoichiometry
I carefully measure 10.00 mL of a solution that
contains an unknown amount of NaCl. It
takes 23.45 mL of the AgNO3 solution to get
the reddish endpoint that shows all the NaCl
has reacted. The concentration of the
AgNO3 solution is 0.09468 M.
What is the concentration of the NaCl solution?
How many grams of NaCl are present in the
sample?
Titrations
Experiments of the type we just worked
through are called titrations, and they are
very important in analytical chemistry.
A titration is a lab procedure in which two
solutions, one of which has a known
concentration, are mixed in carefully
measured amounts just until a chemical
reaction is completed.
Titrations
Careful measurement of the volumes is critical
to good results. We use special glassware
that is usually accurate to 4 sig figs.
Pipets are glass tubes designed to deliver a
known volume of liquid. They are filled by
suction.
Burets are calibrated tubes designed to measure liquid as it is dispensed.
Proper use of these devices will be demonstrated in the laboratory.
Titrations
The analyte is the solution in the flask,
and is usually delivered by pipet. The
analyte contains the limiting reagent
in the titration reaction.
The titrant is the solution in the buret. It
is added until chemical equivalence
is reached.
At chemical equivalence, the analyte is
completely reacted.
Titrations
The endpoint of a titration occurs when chemical equivalence is reached.
There are lots of ways to determine endpoints.
Often, we use indicators like K2CrO4 that
change color. We can also follow electrical
changes in the solution.
It is critical to stop adding solution exactly at the
endpoint (color change). Otherwise, the volume of titrant added will be too large, and
the results inaccurate.
Colligative Properties
of Solutions
Nonvolatile solutes:
Lower the vapor pressure of the solution
relative to that of the pure solvent
Raise the boiling point of the solution
relative to that of the pure solvent
Lower the freezing point of the solution
relative to that of the pure solvent.
Osmosis and
Osmotic Pressure
Osmosis is the passage of solvent
through a semipermeable membrane
from a dilute solution or pure liquid to a
concentrated solution.
Semipermeable membranes have very
small pores, and allow only certain
small molecules to pass from one side
to the other.
A semipermeable membrane separating
(a) pure water and solution
(b) dilute solution and contrated solution
(a) Osmosis, the flow of solvent through a semipermeable membrane from a dilute to a concentrated solution.
(b) At equilibrium, solvent molecules move back
and forth at equal rates.
Osmotic pressure is
the amount of pressure
needed to prevent the
solution in the tube from
rising as a result of the
process of osmosis.
Isotonic, Hypertonic, and Hypotonic Solutions
Isotonic solutions have the same
osmotic pressure as intracellular fluids
Red blood cells are stable
Hypertonic solutions have higher
osmotic pressure that intracellular fluids
Red blood cells undergo crenation
Hypotonic solutions have lower osmotic
pressure than intracellular fluids
Red blood cells undergo hemolysis
Isotonic, Hypertonic, and Hypotonic Solutions
Dialysis
Dialysis is the process in which a semipermeable membrane permits the
passage of solvent and small solute
particles (ions and molecules) but
blocks the passage of large molecules.
Cell membranes permit dialysis.
In dialysis, there is a net movement of ions from a
region of higher concentration to a region of
lower concentration.
(a) Before dialysis.
(b) After dialysis.
Impurities (ions) can be removed from a solution
by using a dialysis procedure.
Reverse Osmosis
Reverse osmosis is a process in which water (or other
solvent) is purified by forcing it through a semipermeable membrane under pressure. It’s used for desalination of water.
Reverse Osmosis
A diagram of a reverse osmosis filter. The membrane
is delicate and needs reinforcement against the
solvent pressure.
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