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					                                CHEMISTRY 152
                                            Exam III

                                           Spring, 1997


All notes, books, etc., must be placed out of sight. Please read each of the problems carefully. If
something is not clear, please ask.

The exam consists of 10 problems on 5 pages. Make sure your exam is complete. The exam is
worth 100 points with each of the problems labeled as to its point value.

Answers should be placed in the space provided and written legibly. If I cannot read it--it is
wrong. You should show your work clearly for each of the problems. Please include
explanations whenever asked or whenever necessary to make your answer clear. Please put your
name on every page of the exam.

There is scratch paper and periodic tables available for your use. If you require more data, ask.

As usual, no cheating will be tolerated.

Data will be found on the next page.
Kw = 1.0 x 10-14                                         Ksp(Fe(OH)2) = 7.94 x 10-16
Ka(NH4+) = 5.6 x 10-10                                   Ksp(Ca(OH)2) = 5.50 x 10-6
Kb(NH3) = 1.8 x 10-5                                     Ksp(Mg(OH)2) = 1.82 X 10-11
Ka(HAc) = 1.8 x 10-5                                     Ksp(Fe(OH)3) = 3.00 x 10-39
Kb(Ac-) = 5.6 x 10-10                                    Ksp(Al(OH)3) = 2.00 x 10-31
                                                         Ksp(AgCl) = 1.8 x 10-10
Ksp(PbI2) = 8.7 x 10-9                             Ksp(AgBr) = 5.0 x 10-13
Ksp(PbCO3) = 1.5 x 10-13                                 Ksp(AgI) = 1.0 x 10-16

       Indicator                            pH Range                   Color Change

       Thymol Blue                   1.2  2.8                         red  yellow
       Methyl Yellow                 2.5  4.5                         yellow  blue
       Bromophenol Blue              3.0  4.6                         yellow  blue
       Congo Red                     3.0  5.0                         blue  red
       Methyl Orange                 3.2  4.4                         red  yellow
       Bromcresol Green              4.0  5.0                         yellow  blue
       Methyl Red                    4.8  6.0                         red  yellow
       Bromcresol Purple             5.2  6.8                         yellow  purple
       Bromthymol Blue               6.0  7.0                         yellow  blue
       Cresol Red                    7.0  8.8                         yellow  red
       Phenolphthalein               8.2  10.0                        colorless  pink
       Thymol Blue                   8.5  9.5                         yellow  blue
       Thymolphthalein               9.3  10.5                        colorless  blue
       Alizarin Yellow R           10.3  11.3                         yellow  red

Representative Weak Acids
HNO2          Nitrous acid                  5.01 x 10-4
HF            Hydrofluoric acid             3.55 x 10-4
HCHO2         Formic acid                   1.82 x 10-4
C6H5CO2H      Benzoic acid                  6.46 x 10-5
HC2H3O2       Acetic acid                   1.78 x 10-5
H2S           Hydrosulfuric acid            8.91 x 10-8
HClO          Hypochlorous acid             3.02 x 10-8
H3BO3         Boric acid                    7.24 x 10-10
HCN           hydrocyanic acid              4.00 x 10-10
Chemistry 152          Exam III       Page 1         Name

1.(5)   A 0.025 M solution of hydroxylamine has a pH of 9.11. What is the value of Kb for this
        weak base? Show work clearly for full credit.

                H2NOH (aq) + H2O (l)  H3NOH+(aq) + OH-(aq)

2.(5)   To make up an unknown solution for the students in your laboratory, you dissolve
        1.0 x 10-5 mole of AgNO3 in 1.0 liter of water. Unfortunately, you used a liter of tap
        water instead of distilled water. If the chloride ion concentration in tap water is
        2.0 x 10-4 M, is your error revealed immediately by the formation of a white precipitate of
        AgCl? Show work to support your answer.
Chemistry 152         Exam III        Page 2         Name

3.(15) You titrate 25.0 ml of 0.0256 M aqueous solution of aniline, C6H5NH2 (MW = 93), a
        weak base with 0.0195 M HCl. Kb (aniline) = 4.0 x 10-10
a. Calculate the pH of the aniline solution before the titration. Show all work clearly for each
    part of this problem.

b. Determine the pH at the equivalance point of the titration.

c. Select an indicator from the data list that would be suitable. Explain your choices.
Chemistry 152          Exam III       Page 3          Name

4.(10) About this time, you may be wishing you had an aspirin. Aspirin (MW = 180) is an
       organic acid with a Ka = 3.27 x 10-4 for the reaction shown below. You have two tablets,
       each having 0.325 grams of aspirin (mixed with a neutral “binder” to hold the tablet
       together), which you dissolve in a glass of water (200 ml). What is the pH of this
       solution? Show all work clearly.

                HC9H7O4 (aq) + H2O (l)  C9H7O4-(aq) + H3O+(aq)

5.(10) A solution contains 0.10 M iodide ion, I-, and 0.10 M carbonate ion, CO3-2.

A. If solid Pb(NO3)2 is slowly added to the solution, which salt precipitates first, PbI2 or PbCO3?
    Convince me of your answer.

B. What is the concentration of the first ion that precipitates (CO3-2, or I-) when the second, or
   more soluble, salt begins to precipitate? Show your work clearly for full credit.
Chemistry 152         Exam III      Page 4         Name

6.(10) A saturated solution of Mg(OH)2 has a pH of 10.49. Use this information to calculate the
       Ksp of Mg(OH)2. Show your work clearly for full credit.

7.(10) Sodium cyanide is the salt of the weak acid HCN. Calculate the concentration of H3O+,
       OH-, HCN, and Na+ in a solution prepared by dissolving 10.8 grams of NaCN (MW = 49)
       in 500 ml of pure water at 25C. Show all of your work clearly.

8.(16) What is the pH change when 0.010 mole of NaOH is added to 100.0 ml of a buffer
       solution consisting of 0.169 M NH3 and 0.183 M NH4Cl? Assume no volume change
       upon the addition. Show your work clearly for full credit.
Chemistry 152          Exam III        Page 5       Name

9.(10) Nicotine, C10H14N2, (abbreviated Nic below) has two basic nitrogen atoms, and both can
       react with water to give a basic solution as shown below. If Kb1 = 7.0 x 10-7, and Kb2 =
       1.1 x 10-10 determine the pH and [NicH2+2(aq)] of a 0.50 M nicotine solution. Show all of
       your work clearly for full credit.

Nic (aq) + H2O (l)  NicH+(aq) + OH-(aq)            NicH+(aq) + H2O (l)  NicH2+2(aq) + OH-(aq)

10.(9) Does the pH of a solution increase, decrease or stay the same when you:

A. Add solid ammonium chloride to a dilute aqueous solution of NH3?

B. Add solid sodium acetate to a dilute aqueous solution of acetic acid?

C. Add solid KCl to a dilute aqueous solution of KOH?

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