Transcript Solutions

Solutions
The Nature of Aqueous
Solutions
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A solution is a homogeneous mixture.
The substance that is dissolved is the
solute.
The substance that does the dissolving
is the solvent. (Water is known as the
universal solvent due to its ability to
dissolve a wide range of solutes)
How do solutes dissolve?
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Ionic solids are all soluble in water.
The “positive” ends of a water molecule are
attracted to the anion of the solid.
The “negative” ends of a water molecule are
attracted to the cation of the solid.
This process is called hydration and results
in the ionic solid being pulled apart, or
dissolved.
What is an electrolyte?
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Solutions that conduct an electric
current efficiently contain strong
electrolytes.
Strong electrolytes are substances that
completely ionize in water.
Three types of compounds are
classified as strong electrolytes.
Strong Electrolytes
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The following are classified as strong
electrolytes:
– Soluble salts (see the handout of the
solubility rules)
– Strong acids (HCl, HBr, HI, H2SO4, HNO3,
HClO3, HClO4)
– Strong bases (group 1A hydroxides, and
Ba(OH)2
What is an acid?
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According to the Arrhenius definition,
an acid is any substance that produces
H+ ions when it is dissolved in water.
Strong acids are those acids in which
every molecule ionizes into H+ ions
and anions.
What is a strong base?
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According to the Arrhenius definition,
a strong base is any soluble ionic
compound containing the hydroxide
ion (OH-).
When these compounds dissolve in
water, the cations and OH- ions
separate and move independently.
What is a weak
electrolyte?
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Weak electrolytes are substances that
exhibit a small degree of ionization in water
and therefore conduct an electric current
inefficiently.
The most common weak electrolytes are
weak acids and weak bases.
An example of a weak acid is acetic acid
(HC2H3O2). An example of a weak base is
ammonium hydroxide (NH4OH)
What is a nonelectrolyte?
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Nonelectrolytes are substances that dissolve
in water but do not produce any ions.
Organic/covalent molecules are
nonelectrolytes. Table sugar (C12H22O11) is
an example of a soluble compound that is a
nonelectrolyte. This is because, when
dissolved, the sugar molecule remains
intact.
Concentration
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Concentrations of solutions are
expressed qualitatively and
quantitatively
Qualitative expressions include the
terms saturated, unsaturated, and
supersaturated.
Concentration
(continued)
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Saturated solution-a solution in which no more
solute will dissolve at a given temperature (the
maximum has been dissolved)
Unsaturated solution-a solution in which more
solute will dissolve at a given temperature (less
than the maximum has been dissolved)
Super-saturated solution-a solution that contains
higher-than-saturation concentration of solute (a
slight disturbance will cause crystallization)
Quantitative Expressions
of Concentration
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Percent by mass
Molarity
Molality
Dilution of solutions
Percent by Mass
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Percent by mass gives the mass of
solute per 100 mass units of solution
Percent solute=mass of solute x 100%
mass of solution
Mass of solution = mass of solute +
mass of solvent
Percent by Mass Problems
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Calculate the mass of NiSO4 contained in 200. g of
a 6.00% solution of NiSO4
A 6.00% solution of NiSO4 contains 40.0 g of NiSO4.
Calculate the mass of the solution.
Calculate the mass of NiSO4 present in 200. mL of a
6.00% solution of NiSO4. The density of the
solution is 1.06 g/mL at 25oC.
What is the volume of a solution that is 15.0%
Fe(NO3)3 and contains 30.0 g of the solute? The
density of the solution is 1.16 g/mL at 25oC.
Molarity
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Molarity-the moles of solute pervolume
of solution in liters.
M = molarity = moles of solute
liters of solution
(solution = solvent + solute)
Molarity Problems
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Calculate the molarity of a solution
prepared by dissolving 11.5 g of solid
NaOH in enough water to make 1.50 L
of solution.
Calculate the mass of Ba(OH)2
required to prepare 2.50 L of a
0.0600 M solution of Ba(OH)2.
Molality
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Molality-the number of moles of solute
per kilogram of solvent
Molality = moles of solute
kilogram of solvent
Molality Problems
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A solution is prepared by mixing 1.00
g of ethanol with 100.0 g of water to
give a final volume of 101 mL. What
is the molality? What is the molarity?
What is the mass percent? What is
the mole fraction?
Dilution of Solutions
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Dilution occurs when more solvent is
added to a solution without changing
the amount of solute that is present.
V1M1 = V2M2
For safety purposes when in a lab
setting, concentrated solutions of acids
and bases are always added slowly to
water.
Dilution Problems
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How many mL of 18.0 M H2SO4 are
required to prepare 1.00 L of a
0.900 M solution of H2SO4?
Review of Solution
Concentration
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The electrolyte in car batteries is a
3.75 M H2SO4 solution that has a
density of 1.230 g/mL. What is the
mass %, molality, and mole fraction of
the sulfuric acid?
Factors Affecting
Solubility
Solubility is the maximum amount of
solute that will dissolve in a given
amount of solvent at a particular
temperature.
Molecular Structure
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The statement “like dissolves like”
summarizes the formation of solutions
based on the structure of the molecules.
“Like dissolves like” means that polar
solvents dissolve ionic and polar solutes
while nonpolar solvents dissolve nonpolar
solutes.
The term miscibility is used to describe the
ability of one liquid to dissolve in another.
(The observation of “like dissolves like” still
applies).
Example of Differentiating
Solvent Properties Based on
Structure
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Decide whether liquid hexane or liquid
methanol is the more appropriate
solvent for the substances grease
(C20H42) and potassium iodide.
Effect of Temperature on
Solubility
The solubility of most solid solutes
increases with increasing temperature.
(exceptions include: Na2SO4 and
Ce2(SO4)3)
 The solubility of gases decreases with
increasing temperature.
(example of this includes thermal
pollution)
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Solubility Graphs
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Solubility curves are graphs that illustrate the effect
of temperature on the solubilities of various salts.
Using the graph on p.496, answer the following
questions:
*How many grams of KNO3 will dissolve in 200 g of
water at 70oC?
*At what temperature will 80 g of KBr dissolve in
100 g of water?
*Which solute’s solubility is least affected by
temperature?
Effect of Pressure on
Solubility
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Changing the pressure has no effect
on the solubilities of either solids or
liquids in liquids.
The solubilities of gases in all solvents
will increase as the partial pressure of
the gases increase.
Henry’s Law
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Henry’s law applies to gases that do
not react with the solvent in which
they dissolve.
The law states that the pressure of a
gas above the surface of a solution is
proportional to the concentration of
the gas in the solution.
Henry’s Law (continued)
The equation for Henry’s law is:
Pgas = kCgas
Pgas is the pressure of the gas above the
solution
k is a constant for a particular gas and solvent
at a particular temperature
Cgas represents the concentration of the
dissolved gas; usually expressed as molarity
or mole fraction.
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Calculations Using
Henry’s Law
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A certain soft drink is bottled so that at 25oC
a bottle contains CO2 gas at a pressure of
5.0 atm over the liquid. Assuming that the
partial pressure of CO2 in the atmosphere is
0.0004 atm, calculate the equilibrium
concentration of CO2 in the soda both
before and after the bottle is opened. The
Henry’s law constant for CO2 at 25oC is
32 L. atm/mol at 25oC. How does your
answer explain why soft drinks go flat after
being opened?
Colligative Properties
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Colligative properties are physical
properties that depend on the number,
not the kind, of solute particles in a
given amount of solvent.
There are four important colligative
properties: vapor pressure lowering,
boiling point elevation, freezing point
depression, osmotic pressure
Lowering of Vapor
Pressure
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Solutions containing a nonvolatile liquid or a
solid as a solute always has a lower vapor
pressure than the pure solvent.
When a solute is dissolved, there are fewer
solvent molecules at the surface of the
liquid.
Therefore, the solvent molecules vaporize at
a slower rate than if no solute were present.
The lowering of the vapor pressure of a
solvent due to the presence of a nonvolatile
solute is summarized by Raoult’s Law.
Raoult’s Law
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The vapor pressure of a solvent in an ideal
solution is directly proportional to the mole
fraction of the solvent in the solution.
Psolvent = Xsolvent P0solvent
The lowering of the vapor pressure can be
expressed as:
ΔPsolvent = XsoluteP0solvent
Sample Problem
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Determine the vapor pressure lowering
of a sucrose solution made by
dissolving 50.0 g of sucrose
(C12H22O11) in 117 g of water. The
vapor pressure of pure water at 25oC
is 23.8 torr.
Sample Problem #2
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Predict the vapor pressure of a
solution made by mixing 35.0 g of
solid Na2SO4 with 175 g water at 25oC.
The vapor pressure of pure water at
25oC is 23.76 torr.
Sample Problem #3
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At 40oC, the vapor pressure of pure
heptane is 92.0 torr and the vapor
pressure of pure octane is 31.0 torr.
Consider a solution that contains 1.00
mole of heptane and 4.00 moles of
octane. Calculate the vapor pressure
of each component and the total vapor
pressure above the solution.
Boiling Point Elevation
A nonvolatile solute elevates the boiling point of
the solvent.
 Boiling occurs at a temperature where the vapor
pressure is equal to 1 atm.
 Addition of a nonvolatile solute lowers vapor
pressure.
 Therefore, a solution must be heated to a higher
temperature than the boiling point of the pure
solvent to reach a vapor pressure of 1 atm.
Equation for Boiling Point
Elevation
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∆T = Kbmsolute
∆T is the boiling point elevation
Kb is the boiling point elevation
constant for the solvent
msolute is the molality of the solute
Calculating Boiling Point
Elevation
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Find the boiling point of a 1.25 m
sucrose solution.
A solution was prepared by dissolving
18.00 g glucose in 150.0 g of water.
The resulting solution was found to
have a boiling point of 100.34oC.
Calculate the molar mass of glucose.
Freezing Point Depression
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When a solute is dissolved in a solvent, the
freezing point of the solution is lower than
that of the pure solvent.
This is the reason that compounds such as
sodium chloride and calcium chloride are
spread on roadways to prevent ice from
forming in freezing weather.
Equation for Freezing
Point Depression
∆T = Kfmsolute
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∆T is the freezing point depression
Kf is the freezing point depression constant
msolute is the molality of the solute
Calculating Freezing Point
Depression
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Calculate the freezing point of a 1.25 m sucrose
solution.
What mass of ethylene glycol (C2H6O2 = 62.1
g/mol), the main component of antifreeze, must be
added to 10.0 L of water to produce a solution for
use in a car’s radiator that freezes at -10.0oF
(-23.3oC)?
A sample weighing 0.546 g was dissolved in 15.0 g
of benzene, and the freezing point depression was
determined to be 0.240oC. Calculate the molar
mass of the sample.
The van’t Hoff Factor
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Colligative properties depend on the number of solute
particles in a given mass of solvent.
The dissociation of electrolytes in aqueous solutions
causes a greater change in these properties than a
solution containing a nonelectrolyte (as in the previous
examples).
One measure of the extent of dissociation of an
electrolyte in water the van’t Hoff factor, i, for the
solution.
i = moles of particles/moles of solute ( or the number of
ions per formula unit)
The observed value of i is always slightly lower than the
predicted value due to ion pairing. (see page 513).
For problem solving purposes, use the ideal value of i
unless the problem indicates otherwise.
Determining the van’t
Hoff factor
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What is the value of the van’t Hoff
factor, i, for the following strong
electrolytes?
a) Na2SO4
b) KOH
c) Al2(SO4)3
d) SrSO4
Properties of Electrolyte
Solutions
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b)
From the following solutions:
0.010 m Na3PO4
0.020 m CaBr2
0.020 m KCl
0.020 m HF in water (HF is a weak acid)
Which would have the same boiling point as
0.040 m C6H12O6 in water?
Which solution would have the highest vapor
pressure at 28oC?
Which solution would have the largest freezing
point depression?
Properties of Electrolyte
Solutions (continued)
From the following:
pure water
0.01 m C12H22O11 in water
0.01 m NaCl in water
0.01 m CaCl2 in water
Choose the one with the
a) Highest freezing point
b) Lowest freezing point
c) Highest boiling point
d) Lowest boiling point
e) Highest vapor pressure
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Solving Problems Using
the van’t Hoff Factor?
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Calculate the freezing point and the
boiling point of each of the following
aqueous solutions.
a) 0.050 m MgCl2
b) 0.050 m FeCl3
Osmotic Pressure
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Osmosis is the passage of a solvent through
a semi-permeable membrane.
Osmotic Pressure is the minimum pressure
that stops osmosis.
The equation for osmotic pressure is
π = iMRT
π is the pressure in atm
M is the molarity of the solution
R = 0.08206 L. atm/K. mol
T is the Kelvin temperature
Calculating Osmotic
Pressure
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To determine the molar mass of a certain
protein, .00100 g of it was dissolved in
enough water to make 1.00 mL of solution.
The osmotic pressure was found to be 1.12
torr at 25.0oC. Calculate the molar mass of
the protein.
What concentration of NaCl in water is
needed to produce an aqueous solution
isotonic with blood (π = 7.70 atm at 25oC)?
Calculating Osmotic
Pressure
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What osmotic pressure would 50.0 g
of sucrose in 117 g of water exhibit at
25oC? The density of this solution is
1.34 g/mL.