LEARNING

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LEARNING OBJECTIVES/ASSESSMENT  When you have comp
leted your study of this chapter, you should be ab
le to   1. Classify mixtures as solutions or nons
olutions based on their appearance. (Section 7.1 
Exercise 7.4)   2. Demonstrate your understanding 
of terms related to the solubility of solutes in s
olution.  (Section 7.2  Exercises 7.6 and 7.12)  
 3. Predict in a general way the solubilities of 
solutes in solvents on the basis of molecular pola
rity.   (Section 7.3 Exercise 7.16)   4. Calculat
e solution concentrations in units of molarity, we
ight/weight percent, weight/volume  percent, and v
olume/volume percent.  (Section 7.4 Exercises 7.2
2 b, 7.30 c, 7.34 a, and 7.38 c)   5. Describe how
 to prepare solutions of specific concentration us
ing pure solutes and solvent, or  solutions of gre
ater concentration than the one desired.  (Section
 7.5 Exercises 7.46 and 7.48 b)   6. Do stoichiom
etric calculations based on solution concentration
s.  (Section 7.6 Exercise 7.56)   7. Understand c
olligative solution properties of boiling point, f
reezing point, and  osmotic pressure and how to
determine osmolarity   (Section 7.7 Exercises 7.
64 a  c and 7.74)   8. Describe the characteristi
cs of colloids.  (Section 7.8 Exercise 7.82)   9.
 Describe the process of dialysis, and compare it 
to the process of osmosis.  (Section 7.9 Exercise
 7.84) 
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Mixtures

• Solutions (sometimes called true solutions) are
  homogeneous mixtures of two or more substances in
  which the components are present as atoms,
  molecules, or ions.
• Particles in these solutions are
• too small to reflect light, are transparent
  (always clear but can be colored).
• in constant motion and not settled by the
  influence of gravity.
• Heterogeneous mixtures
• Particles in these solutions are
• Large aggregates, reflect light (and are turbid)
  and do settle out due to gravity
• Colloidal solutions
• Particles in these solutions are
• Large aggregates, reflect light (and are turbid)
  but do not settle out due to gravity (homogeneous
  mixture)

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The dissolving process

• For true solutions

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SOLUTION TERMINOLOGY

• Solutions can be solids, liquids or gases but we
  will deal primarily with liquid solutions.
• Solutions are composed of a solvent (main
  component) and one or more solutes (substance
  that is dissolved in the solvent)
• Solubility the degree to which a solute
  dissolves (insoluble, slightly soluble, soluble,
  very soluble)
• Miscible/immiscible terms that describe whether
  a liquid solute dissolves or not
• Saturated, unsaturated, supersaturated

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SOLUBILITY

• The solubility of a solute is the maximum amount
  of the solute that can be dissolved in a specific
  amount of solvent under specific conditions of
  temperature and pressure.

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EXAMPLES OF SOLUTE SOLUBILITES AT 0C
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EXAMPLES OF SOLUTE SOLUBILITES AT 0C (continued)
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EFFECT OF TEMPERATURE ON SOLUBILITY
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THE SOLUTION PROCESS

• The solution process involves interactions
  between solvent molecules (often water) and the
  particles of solute.
• An example of the solution process for an ionic
  solute in water

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THE SOLUTION PROCESS (continued)

• An example of the solution process for a polar
  solute in water

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THE SOLUTION PROCESS (continued)

• A solute will not dissolve in a solvent if
• the forces between solute particles are too
  strong to be overcome by interactions with
  solvent particles.
• the solvent particles are more strongly attracted
  to each other than to solute particles.
• A good rule of thumb for solubility is like
  dissolves like.
• Polar solvents dissolve polar or ionic solutes.
• Nonpolar solvents dissolve nonpolar or nonionic
  solutes.

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INCREASING THE RATE OF DISSOLVING

• Crush or grind the solute.
• Small particles provide more surface area for
  solvent attack and dissolve more rapidly than
  larger particles.
• Heat the solvent.
• Solvent molecules move faster and have more
  frequent collisions with solute at higher
  temperatures.
• Stir or agitate the solution.
• Stirring removes locally saturated solution from
  the vicinity of the solute and allows
  unsaturated solvent to take its place.

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HEAT AND SOLUTION FORMATION
Solute solvent heat? solution

• Endothermic
• Exothermic

Solute solvent ? solution heat
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SOLUTION CONCENTRATIONS

• Solution concentrations express a quantitative
  relationship about the amount of solute contained
  in a specific amount of solution.
• Concentration units discussed include molarity
  and percentage.

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MOLARITY

• The molarity of a solution expresses the number
  of moles of solute contained in one liter of
  solution.
• The mathematical calculation of the molarity of a
  solution involves the use of the following
  equation
• In this equation, the number of moles of solute
  in a sample of solution is divided by the volume
  in liters of the same sample of solution.

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PERCENT CONCENTRATIONS

• Percent concentrations express the amount of
  solute contained in 100 parts of solution. The
  parts of solution may be expressed in different
  units.
• Three variations exist for concentrations.
  W/W, W/V or V/V generally expressed in grams and
  mL if not then at least the same units.
• To make solutions of M, W/V and V/V, the amount
  of solute is measured out and solvent added to
  the volume needed.
• To make solutions of W/W , the grams of solvent
  and solute are added to determine the weight of
  the solution.

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CONCENTRATON CALCULATIONS

• Example 1 A 250-mL sample of solution contains
  0.134 moles of solute. Calculate the molarity of
  the solution.
• 9.45 g of methyl alcohol, CH3OH, was dissolved in
  enough pure water to give 500 mL of solution.
  What was the molarity of the solution?
• Calculate the (w/w) of a solution prepared by
  dissolving 15.0 grams of table sugar in 100 mL of
  water. The density of the water is 1.00 g/mL.
• Calculate the (w/v) of a solution prepared by
  dissolving 8.95 grams of sodium chloride in
  enough water to give 50.0 mL of solution.
• A solution is made by dissolving 250 mL of
  glycerin in enough water to give 1.50 L of
  solution. Calculate the (v/v) of the resulting
  solution

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SOLUTION PREPARATION

• Solutions of known concentration are usually
  prepared in one of two ways.
• In one method, the necessary quantity of pure
  solute is measured using a balance or volumetric
  equipment. The solute is put into a container
  and solvent, usually water, is added until the
  desired volume of solution is obtained.

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SOLUTION PREPARATION EXAMPLE

• Calculation example Describe how to prepare 500
  mL of 0.250 M NaCl solution.
• Solution The mass of NaCl needed must first be
  determined. The volume and concentration of the
  desired solution are known, so the equation for
  molarity is rearranged to solve for the number of
  moles of solute needed. The result is
• moles of solute M x liters of solution
  0.250 M x
  0.500 L 0.125 mole
• Thus, 0.125 moles of NaCl is needed. NaCl has a
  formula weight of 58.4 u, so 0.125 moles has a
  mass of 0.125 x 58.4g or 7.30 grams. The
  solution is prepared by weighing a sample of NaCl
  with a mass of 7.30 grams. The sample is put
  into a 500 mL volumetric flask and pure water is
  added up to the mark on the flask.

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SOLUTION PREPARATION (continued)

• In a second method, a quantity of solution with a
  concentration greater than the desired
  concentration is diluted with an appropriate
  amount of solvent to give a solution with a lower
  concentration. This type of problem is made
  simpler by using the following equation
• (Cc)(Vc) (Cd)(Vd)
• In this equation, Cc is the concentration of the
  concentrated solution that is to be diluted, Vc
  is the volume of concentrated solution that is
  needed, Cd is the concentration of the dilute
  solution, and Vd is the volume of dilute solution.

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SOLUTION PREPARATION EXAMPLE

• Calculation example Describe how to prepare 250
  mL of 0.500 M HCl solution from a 1.50 M HCl
  solution.
• Solution According to the definitions given
  above, Cc 1.50 M, Cd 0.500 M, and Vd 250
  mL. The equation given above can be solved for
  Vc, the volume of concentrated solution needed
• The solution is prepared by measuring 83.3 mL of
  1.50 M HCl and pouring it into a 250 mL
  volumetric flask. Pure water is then added up to
  the mark on the flask to give 250 mL of 0.500 M
  solution.

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SOLUTION STOICHIOMETRY

• As shown earlier, the number of moles of solute
  in a volume of solution of known molarity can be
  obtained by multiplying together the known
  molarity and the solution volume in liters.
• Molarity is a ratio of moles of solute to liters
  of solution. This ratio can be written as two
  conversion factors
• The conversion factor on the left is used to
  multiply by the molarity. It is selected to
  cancel the units of liters of solution and obtain
  the units of moles of solute.
• The conversion factor on the right is used to
  divide by the molarity. It is selected to cancel
  the units of moles of solute and obtain the units
  of liters of solution.

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SOLUTION STOICHIOMETRY EXAMPLE

• Calculation example Consider the balanced
  equation
• HCl(aq) NaOH(aq) NaCl(aq)
  H2O(l)
• How many mL of 0.100 M HCl solution would
  exactly react with 25.00 mL of 0.125 M NaOH
  solution?

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SOLUTION PROPERTIES

• Absolutely pure water conducts electricity very
  poorly.
• Some solutes called electrolytes produce water
  solutions that conduct electricity well.
• Some solutes called nonelectrolytes produce water
  solutions that do not conduct electricity.

A solution of a strong electrolyte conducts
electricity well.
A solution of a weak electrolyte conducts
electricity poorly.
A solution of a nonelectrolyte does not conduct
electricity.
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ELECTROLYTES

• STRONG ELECTROLYTES
• Strong electrolytes form solutions that conduct
  electricity because they dissociate completely
  into charged ions when they dissolve.
• WEAK ELECTROLYTES
• Weak electrolytes form weakly conducting
  solutions because they dissociate into ions only
  slightly when they dissolve.
• NONELECTROLYTES
• Nonelectrolytes form nonconducting solutions
  because they do not dissociate into ions at all
  when they dissolve.

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COLLIGATIVE PROPERTIES OF SOLUTIONS

• Colligative solution properties are properties
  that depend only on the concentration of solute
  particles in the solution. Three colligative
  properties are boiling point, freezing point, and
  osmotic pressure.
• Experiments demonstrate that the vapor pressure
  of water (solvent) above a solution is lower than
  the vapor pressure of pure water.

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SOLUTION BOILING POINT

• The boiling point of a solution is always higher
  than the boiling point of the pure solvent of the
  solution.
• Since BP elevation is a colligative property,
  dependent only on the concentration of particles,
  the following substances would have an equal
  impact on BP elevation (or any other colligative
  property)
• 0.1 M CaCl2
• 0.15 M NaCl
• 0.3 M sugar (non-electrolyte)

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SOLUTION BOILING POINT (continued)

• For example, the dissociation of calcium chloride
  is represented as
• CaCl2 Ca2 2 Cl-
• Thus, when 1 mole of CaCl2 dissolves, 3 moles of
  particles (ions) are put into the solution.

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SOLUTION FREEZING POINT

• The freezing point of a solution is always lower
  than the freezing point of the pure solvent of
  the solution.

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OSMOTIC PRESSURE OF SOLUTIONS

• When solutions having different concentrations of
  solute are separated by a semipermeable membrane,
  solvent tends to flow through the membrane from
  the less concentrated solution into the more
  concentrated solution in a process called
  osmosis.
• When the more concentrated solution involved in
  osmosis is put under sufficient pressure, the net
  osmotic flow of solvent into the solution can be
  stopped.
• The pressure necessary to prevent the osmotic
  flow of solvent into a solution is called the
  osmotic pressure of the solution and can be
  calculated by using the following equation, which
  is similar to the ideal gas law given earlier
• p nMRT

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OSMOTIC PRESSURE OF SOLUTIONS (continued)

• In this equation, p is the osmotic pressure, n is
  the number of moles of solute particles put into
  solution when 1 mole of solute dissolves, M is
  the molarity of the solution, R is the universal
  gas constant written as 62.4 L torr/K mol, and T
  is the solution temperature in Kelvin.
• The product of n and M is called the osmolarity
  of the solution.

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COLLOIDS

• Colloids are homogeneous mixtures of two or more
  components called the dispersing medium and the
  dispersed phase. The dispersed phase substances
  in a colloid are in the form of particles larger
  than those found in solutions.
• DISPERSING MEDIUM OF A COLLOID
• The dispersing medium of a colloid is the
  substance present in the largest amount. It is
  analogous to the solvent of a solution.
• DISPERSED PHASE OF A COLLOID
• The dispersed phase of a colloid is the substance
  present in a smaller amount than the dispersing
  medium. It is analogous to the solute of a
  solution.

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COLLOID PROPERTIES

• In colloids, the dispersed phase particles cannot
  be seen and do not settle under the influence of
  gravity.
• Colloids appear to be cloudy because the larger
  particles in the dispersed phase scatter light.
• Colloids demonstrate the Tyndall effect in which
  the path of the light through a colloid is
  visible because the light is scattered.

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TYPES OF COLLOIDS
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STABILIZING COLLOIDS

• Substances known as emulsifying agents or
  stabilizing agents are used to prevent some
  colloids from coalescing (e.g. egg yolk in oil
  and water to form mayonnaise, soap/ detergent
  ions forming a charged layer around nonpolar oils
  and greases).

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STABILIZING COLLOIDS (continued)
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DIALYSIS

• Dialysis can be used to separate small particles
  from colloids (e.g. cleaning the blood of people
  suffering from kidney malfunction).

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DIALYSIS

• A dialyzing membrane is a semipermeable membrane
  with larger pores than osmotic membranes that
  allow solvent molecules, other small molecules,
  and hydrated ions to pass through.
• Dialysis is a process in which solvent molecules,
  other small molecules, and hydrated ions pass
  from a solution through a membrane.