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LEARNING

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Title: LEARNING


1
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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) 
3
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)

4
The dissolving process
  • For true solutions

5
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

6
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.

7
EXAMPLES OF SOLUTE SOLUBILITES AT 0C
8
EXAMPLES OF SOLUTE SOLUBILITES AT 0C (continued)
9
EFFECT OF TEMPERATURE ON SOLUBILITY
10
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

11
THE SOLUTION PROCESS (continued)
  • An example of the solution process for a polar
    solute in water

12
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.

13
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.

14
HEAT AND SOLUTION FORMATION
Solute solvent heat? solution
  • Endothermic
  • Exothermic

Solute solvent ? solution heat
15
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.

16
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.

17
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.

18
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

19
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.

20
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.

21
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.

22
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.

23
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.

24
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?

25
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.
26
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.

27
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.

28
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)

29
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.

30
SOLUTION FREEZING POINT
  • The freezing point of a solution is always lower
    than the freezing point of the pure solvent of
    the solution.

31
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

32
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.

33
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.

34
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.

35
TYPES OF COLLOIDS
36
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).

37
STABILIZING COLLOIDS (continued)
38
DIALYSIS
  • Dialysis can be used to separate small particles
    from colloids (e.g. cleaning the blood of people
    suffering from kidney malfunction).

39
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.
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