Chemistry 1312
Exam 2, Spring, 2001

I. Multiple Choice (96 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. A 44.0 mL aliquot of 0.200 M HI (titrant) was added to 25.0 mL of a NaOH solution (analyte) until the phenolphthalein endpoint was reached. What is the molar concentration of the sodium hydroxide solution?
a. 0.176 M NaOH
b. 0.200 M NaOH
c. 0.227 M NaOH
d. 0.352 M NaOH
e. none of the above

2. To 20.0 mL of 0.100 M HBr is added 10.0 mL of 0.080 M KOH. An analysis of the resulting mixture reveals that at equilibrium
a. only Br- is present to affect the pH of the solution
b. only H3O+ is present to affect the pH of the solution
c. both HBr and Br- are present to affect the pH of the solution, but [HBr] > [Br-]
d. both HBr and Br- are present to affect the pH of the solution, but [HBr] < [Br-]
e. only HBr is present to affect the pH of the solution

3. To 20.0 mL of 0.100 M HBrO2 is added 10.0 mL of 0.080 M NaOH. An analysis of the resulting mixture reveals that at equilibrium
a. only BrO2- is present to affect the pH of the solution
b. only H3O+ is present to affect the pH of the solution
c. both HBrO2 and BrO2- are present to affect the pH of the solution, but [HBrO2] > [BrO2-]
d. both HBrO2 and BrO2- are present to affect the pH of the solution, but [HBrO2] < [BrO2-]
e. only HBrO2 is present to affect the pH of the solution

4. At the stoichometric point for the titration of a weak base, B, with a strong acid, HCl,
a. the pH < 7 because of the presence of excess H3O+
b. the pH = 7 because the base is neutralized
c. the pH > 7 because of the presence of excess base, B
d. the pH < 7 because of the presence of the BH+
e. the pH > 7 because of the presence of the conjugate base of a weak acid

5. Consider a cyanic acid, HCNO, solution. A small amount of solid NaCNO is added to the solution.
As a result,
a. the [H3O+] increases
b. the pH decreases
c. the pH increases
d. the [OH-] decreases
e. no change occurs in the [H3O+]

6. The solubility of each of the following slightly soluble salts is pH dependent except
a. FeS
b. SrCO3
c. AgCH3CO2
d. Al(OH)3
e. AgI

7. An aqueous solution is 0.60 M HPr (propionic acid) and 0.30 M NaPr (sodium propionate). What is the consequence of adding 4.00 g NaOH (molar mass = 40.0 g/mol) to one liter of this solution?
a. [Pr-] increases to 0.40 mol/L only
b. [HPr] increases to 0.70 mol/L and [Pr-] decreases to 0.20 mol/L
c. [HPr] decreases to 0.50 mol/L and [Pr-] decreases to 0.20 mol/L
d. [HPr] decreases to 0.50 mol/L and [Pr-] increases to 0.40 mol/L
e. [HPr] decreases to 0.50 mol/L only

8. Consider the slightly soluble salt, Ag2SO4. At equilibrium,
a. the molar solubility of Ag2SO4 equals twice the molar concentration of Ag+
b. the molar solubility of Ag2SO4 equals one-half the molar concentration of SO42-
c. the molar solubility of Ag2SO4 equals the molar concentration of Ag+
d. the molar solubility of Ag2SO4 equals the molar concentration of SO42-
e. the molar concentration of Ag+ equals the molar concentration of SO42-

9. All of the following aqueous solution mixtures result in a buffer solution except one. Which one?
a. 50.0 mL 0.20 M NaCH3CO2 and 50.0 mL 0.30 M CH3COOH
b. 50.0 mL 0.20 M CH3COOH and 50.0 mL 0.30 M NaOH
c. 50.0 mL 0.20 M NH4Cl and 50.0 mL 0.30 M NH3
d. 50.0 mL 0.20 M HCl and 50.0 mL 0.30 M NH3
e. 50.0 mL 0.20 M NaOH and 50.0 mL 0.30 M CH3COOH

10. Of the following salts, all produce a basic solution when dissolved in water except one. Which one?
a. Na2CO3
b. Ba(OH)2
c. KCN
d. K3PO4
e. Mg(NO3)2

II. Salts (12 points) Complete the following table.

Salt         pH =7, >7, <7 Equation (balance for mass and charge!) that justifies pH prediction

NH4Cl            <7 NH4+ + H2O « H3O+ + NH3

Ni(NO3)2     <7 Ni(H2O)2+ + H2O « H3O+ + NiOH+

CaBr2             =7

KCNO             >7  CNO- + H2O « HCNO + OH-

III. 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) Determine the [H3O+] in 250.0 mL solution of a 0.20 M sodium formate solution.
Ka for formic acid = 1.7 x 10-4

[H3O+] = 2.92 x 10-9 mol/L; pOH = 5.46, pH = 8.54

2. (25 points) A 250.0 mL solution is prepared that is 0.36 M formic acid, HCOOH, and 0.28 M sodium formate, NaCHO2. Ka = 1.7 x 10-4
a. (10 pts) What is the pH of this solution?

pH = 3.66

b. (15 pts) To the solution in Part a is added 6.0 x 10-2 mol HCl. What is the change in pH?

DpH = 1.07

3. (15 points) A combination of a 250.0 mL volume of a 0.030 M formic acid, HCOOH, and solid sodium formate, NaCHO2, (molar mass = 68.0 g/mol) is used to prepare a buffer solution with a pH of 4.00.
Ka = 1.7 x 10-4
a. (10 pts) What must be the molar ratio of [CHO2-]/[HCOOH] in the prepared buffer?

[CHO2-]/[HCOOH] = 1.7/1

b. (5 pts) How many grams of sodium formate must be added to the formic acid solution to prepare the buffer?

0.867 g NaCHO2

4. (20 points) In a titration, 35.0 mL of 0.900 M KOH is added to 250.0 mL of a 0.150 M formic acid, HCOOH, solution. Ka = 1.7 x 10-4
a. (15 pts) What is the pH of the resulting mixture?

pH = 4.49

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

41.7 mL of 0.900 M KOH

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