Chemistry 1312
Exam 2, Spring, 2003

I.  Multiple Choice (91 points)  Select the best response for each of the following and record on the scantron.  Do not erase; erasures may result in a misgrading of your response.

1.     At the stoichiometric point for a propionic acid (abbreviated HPr)-sodium hydroxide titration,
a.     the pH > 7 because of the presence of the NaOH
b.     the pH > 7 because of the presence of the Pr^-
c.     the pH = 7 for all stoichiometric points
d.    the pH < 7 because of the presence of the HPr
e.     the pH < 7 because of the presence of the H₂Pr⁺

2.     For a potassium acetate, KCH₃CO₂, solution,
a.     [CH₃CO₂^-] > [CH₃COOH] = [OH^-]
b.     [CH₃CO₂^-] < [CH₃COOH] < [OH^-]
c.     [CH₃CO₂^-] < [CH₃COOH] = [OH^-]
d.    [CH₃CO₂^-] = [CH₃COOH] > [OH^-]
e.     [CH₃CO₂^-] > [CH₃COOH] > [OH^-]

3.     Which of the following salts affect the pH of water?  All of the salts are water soluble.
1.  KCN       2.  NaF        3.  KClO₄       4.  NH₄Br       5.  AgNO₃
a.     2 only
b.     2, 4, 5 only
c.     1, 2, 5 only
d.    1, 2, 4 only
e.     all change the pH of the solution

4.     For the HCO₃^-/CO₃^2- buffer solution, the addition of a small amount of HCl
a.     increases the [HCO₃^-] only
b.     increases the [HCO₃^-] and decreases the [CO₃^2-]
c.     decreases the [HCO₃^-] only
d.    decreases the [HCO₃^-] and increases the [CO₃^2-]
e.     decreases the [CO₃^2-] only

5.     Equal volumes of 0.10 M HCl and 0.10 M NH₃ are mixed.  The final pH of the solution at 25°C is
a.     7
b.     >7
c.     <7
d.    14
e.     unknown, need to know the actual volumes

6.     Which of the following pH curves represents the titration of a weak base with a strong acid?

(five different pH curves are presented for this question)

7.     Consider the HCO₃^-/CO₃^2- buffer system:  which of the following mixtures has the lowest [H₃O⁺]?
a.     0.20 M HCO₃^- and 0.10 M CO₃^2-                           
b.     0.10 M HCO₃^- and 0.10 M CO₃^2-                                         
c.     0.20 M HCO₃^- and 0.20 M CO₃^2-
d.    0.10 M HCO₃^- and 0.20 M CO₃^2-
e.     0.20 M HCO₃^- only

8.     Which of the following indicators would be suitable for the detection of the stoichiometric point in the titration of a benzoic acid, HBz, sample (a weak acid) with a standardized NaOH solution?
                                                                                                pH range of color change
a.     methyl orange                                                            3.2 to 4.4
b.     litmus                                                                            5.0 to 8.0
c.     bromophenol blue                                                     3.0 to 4.6
d.    thymol blue                                                                 8.0 to 9.6
e.     bromothymol blue                                                     6.0 to 7.6

9.     The pH curve for the titration of 25.0 mL of 0.100 M HClO with 0.100 M NaOH is given below.  The main species in solution at point A is/are:
(a weak acid/strong base pH curve is given...point "A" is that at the halfway stoichiometric point)
a.     HClO and OH^-
b.     HClO and ClO^-
c.     OH^- and H₃O⁺
d.    ClO^-
e.     ClO^- and OH^-

10.  Consider the data:
HClO₂                                                                                   K_a = 1.1 x 10^-2
CH₃COOH                                                                           K_a = 1.8 x 10^-5
HCN                                                                                      K_a = 4.7 x 10^-10
HF                                                                                          K_a = 6.7 x 10^-4
Which of the following solutions has the lowest [OH^-]?
a.     0.1 M NaClO₂
b.     0.1 M NaCN
c.     all solutions have the same pH
d.    0.1 M NaCH₃CO₂
e.     0.1 M NaF

11.  The K_sp expression for determining the molar solubility of the slightly soluble salt, Pb₃(AsO₄)₂, where x = molar solublity of Pb₃(AsO₄)₂, is:
a.     K_sp = [3x][2x]
b.     K_sp = [x][x]
c      K_sp = [3x]²[2x]³
d.    K_sp = [3x]³[2x]²
e.     K_sp = [x]³[x]²

12.  Consider the equation for a saturated solution of calcium sulfite:
CaSO₃(s) _«  Ca²⁺(aq)  +  SO₃^2-(aq)
What happens, if anything, when hydrochloric acid is added to the solution?
a.     nothing
b.     the pH of the solution increases
c.     the amount of CaSO₃ increases
d.    the SO₃^2- concentration increases
e.     the Ca²⁺ concentration increases

13.  What must be the [tar^2-]/[Htar^-] ratio for a buffer solution having a pH = 4.00? 
K_a2 for tartaric acid, H₂tar = 1.5 x 10^-5
a.     0.15
b.     1.5
c.     0.67
d.    2.0
e.     6.7

14.  One liter of a buffer solution consisting of 0.060 mol HCOOH and 0.080 mol CHO₂^- is prepared.  What is a consequence of 0.010 mol of NaOH being mistakenly added to the system?  K_a for HCOOH is 1.8 x 10^-4
a.     At equilibrium [HCOOH] ≈ 0.060 mol/L and [CHO₂^-] ≈ 0.090 mol/L
b.     At equilibrium [HCOOH] ≈ 0.050 mol/L and [CHO₂^-] ≈ 0.080 mol/L
c.     At equilibrium [HCOOH] ≈ 0.060 mol/L, [CHO₂^-] ≈ 0.080 mol/L, and [OH^-] ≈ 0.010 mol/L
d.    At equilibrium [HCOOH] ≈ 0.050 mol/L and [CHO₂^-] ≈ 0.090 mol/L
e.     At equilibrium [HCOOH] = [CHO₂^-] ≈ 0.070 mol/L

II. Exercises (75 points).  The organization of all exercises must be shown clearly and legibly in order to receive any partial credit.  Answers without a logical problem setup will receive only 10% credit.

1.     (15 points)  Predict if the following salt solutions will produce a solution with a pH =7, >7, or <7.  If the solution is predicted to have a pH >7 or <7, write a “CORRECT” Brønsted equation that supports your prediction.

Salt	pH = 7, >7, <7	Equation
NaNO2	> 7	NO2-  +  H2O «  HNO2  +  OH-
Ba(ClO4)2	= 7	All spectator ions
CH3NH3Cl	< 7	CH3NH3+  +  H2O «  H3O+  +  CH3NH2

2.     (15 points)  A 50.0 mL aliquot of 0.400 M nitric acid, HNO₃, is titrated with 0.100 M NaOH.
a.     (10 pts)  What is the pH after the addition of 150.0 mL of 0.100 M NaOH?

Strong acid/strong base mixture
pH = 1.60

b.     (5 pts)  What volume of titrant is required to reach the stoichiometric point for the titration?

200 mL

3.     (15 points)  What is the pH resulting from a mixture of 50.0 mL of 0.400 M C₅H₄NCOOH (called nicotinic acid, abbreviate as HNic) and 150.0 mL of 0.100 M KOH?  K_a of C₅H₄NCOOH = 1.4 x 10^-5.
Weak acid/strong base mixture
pH = 5.33

4.     (20 points)  A solution of 500 mL is prepared that is 0.400 M HCOOH and 0.200 M NaHCO₂.  K_a for HCOOH is 1.8 x 10^-4
a.     (10 pts)  What is the pH of the solution?

buffer solution
pH = 3.44

b.     (10 pts)  What is the pH after the addition of 0.150 moles of KOH to the 500 mL of solution?

pH = 4.44

5.     (10 points)  Determine the percent hydrolysis of 0.200 M potassium bromoacetate, KCH₂BrCO₂.  K_a for bromoacetic acid, CH₂BrCOOH, is 1.3 x 10^-3

6.2 x 10^-4 %