Chemistry 1312
Exam 2, Spring 2002

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. Identify the solution having the highest [H3O+].
a. 25.0 mL of 0.10 M HClO2
b. 25.0 mL of 0.10 M HClO2 and 10.0 mL of 0.10 M NaClO2
c. 25.0 mL of 0.10 M HClO2 and 15.0 mL of 0.10 M NaClO2
d. 25.0 mL of 0.10 M HClO2 and 25.0 mL of 0.10 M NaClO2
e. 25.0 mL of 0.10 M HClO2 and 30.0 mL of 0.10 M NaClO2

2. For the titration of 25.0 mL of 0.010 M N2H5Cl with 0.020 M HCl, the pH at the stoichiometric point will
a. equal 7.0 at 25°C
b. be less than 7.0 because of the titrant being HCl
c. be greater than 7.0 because of the analyte being N2H5Cl
d. be less than 7.0 because of the presence of N2H5+
e. be greater than 7.0 because of the presence of the conjugate base of a weak acid

3. An aqueous mixture is formed from 10.0 mL of 0.020 M HCOOH and 10.0 mL of 0.010 M KOH. Ka of HCOOH is 1.8 x 10-4. Identify the one true statement: The pH of the resulting mixture is…
a. 2.00
b. 3.74
c. 7.00
d. 1.70
e. 11.00

4. The addition of acid is often used in an attempt to dissolve a precipitate. An acid addition increases the solubility of all of the following salts except
a. PbI2
b. Ag2C2O4
c. Ca3(PO4)2
d. Fe(OH)3
e. MnS

5. All of the following solution mixtures result in the formation of a buffer solution except
a. 50.0 mL of 0.10 M HNO2 and 25.0 mL of 0.10 M KOH
b. 25.0 mL of 0.10 M HClO4 and 50.0 mL of 0.10 M KNO2
c. 25.0 mL of 0.10 M HNO2 and 50.0 mL of 0.10 M KNO2
d. Equal volumes of 0.10 M HNO2 and 0.10 M NaOH
e. Equal volumes of 0.10 M HNO2 and 0.10 M KNO2

6. Identify the pair of salts that, when separately dissolved in water, produce solutions with a pH greater than 7.0.
a. KClO4, KNO2
b. (CH3)2NH2NO3, NH4NO3
c. CoCl3, Fe(NO3)3
d. K2C2O4, KClO
e. Ba(NO3)2, AlCl3

7. What must be the [Cit3-]/HCit2-] ratio for a buffer solution having a pH = 7.00?
Ka3 for citric acid, H3Cit = 4.0 x 10-7
a. 0.25
b. 0.50
c. 1.00
d. 2.0
e. 4.0

8. How would you increase the buffer capacity of a NaHCO3, Na2CO3 buffer system?
a. increase the concentration of NaHCO3
b. increase the concentration of Na2CO3
c. increase the concentrations of NaHCO3 and Na2CO3
d. increase the ratio of CO32- to HCO3-
e. decrease the concentrations of NaHCO3 and Na2CO3

9. Identify the one ion that is not a conjugate base of a weak acid and does not produce a basic solution.
a. S2-
b. CN-
c. OH-
d. HSO4-
e. IO3-

10. The conjugate base of the Co3+ ion in an aqueous system is
a. CoOH2+
b. Co(H2O)3+
c. Co(H2O)63+
d. Co2+
e. Co(OH)2+

11. What is the effect of the addition of KIO3 to a saturated Ba(IO3)2 solution? Ksp of Ba(IO3)2 = 1.6 x 10-9
a. the [K+] and [Ba2+] increase
b. only the [Ba2+] increases
c. more Ba(IO3)2 precipitates
d. more Ba(IO3)2 dissolves
e. no changes are predicted

12. Referring to the pH curves, which acid has the smallest Ka value? (pH CURVES NOT SHOWN)
a. CH2ClCOOH
b. HOCl
c. CH3COOH
d. all have the same Ka
e. depends upon the molar concentrations

13. Referring to the pH curves, the endpoint for the indicator used to detect the stoichiometric point for the titration of CH3COOH should occur at (pH CURVES NOT SHOWN)
a. pH = 5.0
b. pH = 7.0
c. pH = 9.0
d. pH = 10.0
e. pH = 11.0

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
LiI

=7

 No equation, only "spectator" ions
C5H5HNO3

<7

 C5H5NH+ + H2O « H3O+ + C5H5N
K2C3H2O4

>7

 C3H2O42- + H2O « HC3H2O4- + OH-

2. (15 points) A 10.0 mL aliquot of 0.150 M nitric acid, HNO3, is titrated with 0.050 M KOH.
a. (10 pts) What is the pH after the addition of 35.0 mL of 0.050 M KOH?

pH = 11.74

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

30.0 mL

3. (10 points) A 500 mL volumetric flask of 0.040 M sodium pyruvate, NaC3H3O3, was found in an old storage cabinet. What is the suspected pH of this solution? Ka for HC3H3O3= 3.2 x 10-3

pH = 7.55

4. (15 points) A 20.0 mL aliquot of 0.140 M glycolic acid, HC3H3O2, is titrated with 0.40 M KOH.
Ka for HC3H3O2 = 1.5 x 10-4
a. (5 pts) What volume of titrant is required to reach the stoichiometric point for the titration?

7.00 mL

b. (10 pts) What is the pH at the stoichiometric point of the titration?

pH = 8.42

5. (20 points) A 250 mL volume of a solution is 0.70 M NaHC4H2O4 (sodium hydrogen fumarate) and 0.30 M Na2C4H2O4 (sodium fumarate). Ka2 for H2C4H2O4 (fumaric acid) is 3.2 x 10-5
a. (10 pts) What is the pH of the mixture?

pH = 4.13

b. (10 pts) What is the pH after the addition of 0.080 mol NaOH(g) to the mixture? Assume no volume change.

pH = 4.71

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