Lecture

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Lecture 26Whats on the Final?

• Chemistry 142 B
• Autumn Quarter, 2004
• J. B. Callis, Instructor

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General

• Date as Announced in Syllabus
• 44 Multiple-Choice Questions - Chapters 1-8
• 22 questions from chapters 1-5
• 22 questions from chapters 6-8
• Practice exam available on the class web site
  key on class bulletin board.

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Chapter 2Daltons Atomic Hypothesisand Beyond

• Law of multiple proportions as explained by
  Dalton
• Naming compounds

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Problem 26-1 Multiple Proportions
A chemist studies three oxides of iodine, and
finds their oxygen as follows 20.14 , 23.97
and 22.10 . (a) Calculate the mass of oxygen per
gram of iodine in each compound. Express the
result as a ratio. Then form the ratio of ratios
by dividing by the ratio of the first compound.
(b) How do the numbers in part (a) support
Daltons atomic theory?
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Problem 26-1 Multiple Proportions
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Chapter 3 - Stoichiometry

• Convert between moles, mass and number of
  molecules.
• Determine empirical formula from elemental
  composition.
• Balance a chemical reaction using the atom
  balance equations.
• Solve stoichiometric problems involving a
  limiting reagent using the reactant ratio method.

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Problem 26-2 Bromoform is 94.85 bromine, 0.40
hydrogen and 4.75 carbon by mass. Determine its
empirical formula.
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Problem 26-3 Balance the following chemical
equation x NaCl y SO2 z H2O w O2 gt
u Na2SO4 v HCl
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Problem 26-4 Sulfuric acid (H2SO4) forms in the
following reaction 2 SO2 O2 2 H2O gt 2
H2SO4 Suppose 400. g SO2, 175. g O2 and 125. g
H2O are mixed and the reaction proceeds until one
of the reactants is used up. Identify the
limiting reactant and determine what masses of
the other reactants remain.
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Chapter 4Chemical Reactions

• Classify a given reaction and identify products
  such as precipitates.
• Write the net ionic reaction.
• Determine the oxidation number of an element in a
  compound.
• Balance a redox equation using the atom balance
  and charge balance equations.
• Perform stoichiometric calculations on solutions

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Problem 26-5 Balance the following chemical
equation x H y H2O2 z Fe2 gt u Fe3
v H2O
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Chapter 5 The Gas Phase

• Use the ideal gas law to compute density from
  other independent variables, e.g. pressure,
  temperature.
• Work with partial pressures.
• Calculate the amount of gas collected over water.
  
• Use the ideal gas law in stoichiometric problems.

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Problem 26-6 A 20.6 L sample of air is collected
in Greenland at 20. oC at a pressure of 1.01 atm
and forced into a 1.05 L bottle for shipment to
Europe for analysis. Compute the pressure inside
the bottle as it is opened in the laboratory at
21. oC.
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Chapter 6 Chemical Equilibrium

• Write the equilibrium constant for a balanced
  chemical reaction.
• Solve equilibrium problems given starting
  concentrations and K.
• Apply Le Chateliers principle to determine
  direction of shift of reaction in response to a
  change in reaction conditions.

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Problem 26-7 At 5000 K, even the nitrogen
molecule (N2) breaks down (into 2 atoms of
nitrogen). At this temperature, when the total
pressure of nitrogen is 1.00 atm, N2(g) is a
0.65 dissociated at equilibrium N2(g) 2
N(g). Compute the equilibrium constant at 5000K.
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Problem 26-7 (cont.)
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Chapter 7 Acid/Base Equilibria

• Understand the pH scale
• Calculate the pH of strong, weak and very weak
  acids and bases

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Problem 26-8 What is the pH of 0.15 M
methylammonium bromide, CH3NH3Br? (Kb of CH3NH2
4.4 x 10-4
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Chapter 8 Applications of Aqueous Equilibria

• Calculate the pH of buffer solutions
• Give recipes for making up buffers
• Calculate pH at different points in a titration
  curve
• Perform solubility calculations
• Understand the basic ideas of complex ion
  equilibria

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Problem 9 You have at your disposal an ample
quantity of a solution of 0.0500 M NaOH and 500
mL of a solution of 0.100 M formic acid (HCOOH,
Ka 1.77 x 10-4). How much of the NaOH solution
should be added to the acid solution to produce a
buffer of pH 4.00?
Ans Use the base to produce a sufficient amount
of the formate ion to provide a buffer of the
desired pH. Do the problem at the
Henderson-Hasselbalch level, ignoring the
ionization of water. Allow for dilution of the
original acid solution with the base, just as in
a titration. The relevant reaction is
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Problem 9 (cont.)
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Problem 9 (cont.)
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Problem 10 A saturated solution of Mg(OH)2 at
25oC is prepared by equilibrating solid Mg(OH)2
with water. Concentrated NaOH is then added until
the solubility of Mg(OH)2 is 0.001 times that in
H2O alone. (Ignore the change in volume resulting
from the addition of NaOH.) The solubility
product Ksp of Mg(OH)2 is 1.2 x 10-11 at 25oC.
Calculate the concentration of hydroxide ion in
the solution after the addition of the NaOH.
Ans First calculate the solubility of Mg(OH)2 in
water. Then calculate the concentration of Mg2
after addition of OH- put this into the mass
action expression and solve for OH-.
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Problem 10 (cont.)
Addition of OH- shifts the equilibrium to the
left and the concentration of Mg2 must diminish
to maintain the mass-action expression at a
constant value. After the addition of base, the
new concentration of Mg2 is
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Answers to Questions from Lecture 26

• 1, 1.25, 1.125
• CHBr3
• 4 NaCl 2 SO2 2 H2O O2 gt 2 Na2SO4 4
  HCl
• 2 H H2O2 2 Fe2 gt 2 Fe3 2 H2O
• 23.0 atm
• K 1.67 x 10-4
• pH 5.73
• 639 mL
• 9.1 x 10-3