Chemistry 1312, Exam 2
Spring, 1999

I. Multiple Choice (80 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 salt that produces an acidic solution is
a. K₂CO₃
b. NH₄NO₃
c. AgCH₃CO₂
d. Ba(OH)₂
e. Na₃PO₄

2. Identify the salt that produces an aqueous solution with the highest pH.
a. KCN, K_a for HCN = 7.2 x 10^-10
b. LiHCO₂, K_a for HCOOH = 1.8 x 10^-4
c. NaF, K_a for HF = 7.2 x 10^-4
d. KOCl, K_a for HOCl = 3.0 x 10^-8
e. all salts have a pH = 7

3. Equal volumes of the following solutions were mixed. Which mixture does not result in a buffer solution?
a. 0.010 M HNO₂ and 0.020 M KNO₂
b. 0.020 M HNO₂ and 0.010 M KOH
c. 0.010 M HNO₂ and 0.010 M KOH
d. 0.020 M HNO₂ and 0.010 M KNO₂
e. all of the mixtures are buffer solutions

4. Select the statement that is true for an aqueous sodium phosphate, Na₃PO₄, solution?
a. Na⁺, PO₄^3-, HPO₄^2-, and OH^- ions are in solution
b. the Na⁺ ion is of highest concentration
c. the concentration of the HPO₄^2- equals that of OH^-
d. the pH of the solution is greater than 7
e. all of the statements are true

5. Equal volumes of equal concentrations of a molecular weak base and its conjugate acid cation are present in solution. The K_b for the weak base is 1.0 x 10^-6. A "small" amount of HCl(aq) is added to the solution. Which of the following statements is false?
a. The pH of the solution decreases
b. The concentration of the molecular weak base increases
c. The concentration of the acid cation increases
d. The ratio of increases
e. The concentration of the hydroxide ion decreases

6. The K_b for hydroxylamine, NH₂OH, is 1.1 x 10^-8. All of the following are true for a 0.10 M NH₂OH solution except
a. The pH of the solution is less than 7
b. [OH^-] > [H₃O⁺] at equilibrium
c. NH₂OH is the substance in highest concentration
d. [NH₃OH⁺] = [OH^-] at equilibrium
e. [NH₂OH] > [NH₃OH⁺] at equilibrium

7. Consider the equilibrium system for benzoic acid, C₆H₅COOH(aq) + H₂O(l) <--> H₃O⁺(aq) + C₆H₅CO₂^-(aq)
K_a for benzoic acid is 6.3 x 10^-5. Which of the following statements is false?
a. Addition of OH^- increases the concentration of C₆H₅CO₂^-
b. Removing C₆H₅COOH increases the pH of the solution
c. Increasing the H₃O⁺ concentration increases the C₆H₅CO₂^- concentration
d. Increasing the C₆H₅COOH concentration increases the C₆H₅CO₂^- concentration
e. Increasing the C₆H₅CO₂^- concentration increases the pH of the solution

8. What can be concluded after 10.0 mL of 0.10 M KOH is added to 20 mL of 0.10 M CH₃COOH? K_a for CH₃COOH is 1.8 x 10^-5
a. K_a = [H₃O⁺]
b. The halfway titration point is reached
c. The pH of the solution is less than 7
d. [CH₃COOH] = [CH₃CO₂^-]
e. all statements are true

9. The music played in the lecture hall, N251, prior to the Chem 1312 lecture is typically
a. Rap
b. Jazz
c. Tejano
d. Country
e. Classical

10. The pH of a 0.10 M HNO₂ solution is 2.18. What is the percent dissociation of the HNO₂ is solution?
a. 6.6%
b. 10.0%
c. 100%
d. 2.18%
e. need to know the K_a of HNO₂

11. The pH at the stoichiometric point of a HClO-KOH titration is
a. >7, because of the presence of KOH at the stoichiometric point
b. =7, because KOH neutralizes HClO at the stoichiometric point
c. >7, because of the buffering action of the resulting HClO/ClO^- mixture
d. >7, because of the presence of the ClO^- at the stoichiometric point
e.     <7, because of the buffering action of the resulting HClO/ClO^- mixture

12. Which indicator is the "best" selection for the acid-base titration of 0.10 M pyridine with 0.1 M HCl?
a. methyl orange, pK_In = 3.46
b. bromothymol blue, pK_In = 7.1
c. litmus, pK_In = 6.5
d. phenolphthalein, pK_In = 9.4
e. alizarin, pK_In = 11.7

13. A colony of "bugs" that you want to grow require a buffered media with a pH of 6.2. Which of the following generic acid/conjugate base systems will be adequate for your preparation?
a. HQ/Q^-, K_a = 1.0 x 10^-7
b. HZ/Z^-, K_a = 6.3 x 10^-5
c. HY/Y^-, K_a = 9.9 x 10^-8
d. HA/A^-, K_a = 6.2 x 10^-14
e. HR/R^-, K_a = 1.0 x 10^-6

14. At the stoichiometric point of a HClO-KOH titration, which of the following species are present (besides water)?
a. K⁺ and ClO^- in equal concentrations with lesser, but equal concentrations of H₃O⁺ and OH^-
b. K⁺ and Cl^- in equal concentrations with lesser, but equal concentrations of H₃O⁺ and OH^-
c. K⁺ with lesser, but equal concentrations of ClO^- and OH^-
d. K⁺ and ClO^- in equal concentrations with a lesser concentration of HClO
e. K⁺ with a lesser concentration of ClO^-, a lesser but equal concentrations of HClO and OH^-, and even less concentration of H₃O⁺

15. Which of the following aqueous solution has the highest pH.
a. 0.010 M HCN, K_a for HCN = 7.2 x 10^-10
b. 0.010 M HCOOH, K_a for HCOOH = 1.8 x 10^-4
c. 0.010 M HF, K_a for HF = 7.2 x 10^-4
d. 0.010 M HOCl, K_a for HOCl = 3.0 x 10^-8
e. 0.010 M HNO₃

II. Exercises. (95 points) All work must be shown clearly and legibly in order to receive credit. Answers, without a logical presentation, will receive only 10% credit.

1. (15 points) Predict if the following salt solutions will produce an acidic, basic, or neutral solution. If acidic or basic, write a "correct" equation that supports your prediction.

2. (15 points) A buffer solution with a pH of 12.0 is to be prepared from solid Na₂HPO₄ (molar mass = 142 g/mol) and Na₃PO₄ (molar mass = 164 g/mol). What must be the mass ratio of Na₃PO₄ to Na₂HPO₄ for the preparation of the buffer solution? K_a for HPO₄^2- is 4.8 x 10^-13
0.55 g Na₃PO₄/1 g Na₂HPO₄

3. (15 points) What is the concentration of hydroxide ion and the pH of 0.070 M sodium phenoxide, NaPh? K_a of phenol (HPh) is 1.3 x 10^-10.
[OH^-] = 2.3 x 10^-3 mol/L; pH = 11.37

4. (15 points) A 25.0 mL volume of an aqueous solution of 0.40 M C₂H₅COOH (propionic acid) is titrated with a 25.0 mL volume of 0.30 M KOH. What is the pH of the resulting solution?   K_a for propionic acid is 1.4 x 10^-5
pH = 5.33

5. (20 points) A 500 mL volume of an aqueous solution is 0.40 M C₂H₅COOH (propionic acid) and 0.10 M NaC₂H₅CO₂. K_a for propionic acid is 1.4 x 10^-5
a. (10 pts) What is the pH of the solution mixture?
pH = 4.25
b. (10 pts) What is the pH of the solution mixture after the addition of 0.050 moles of KOH?
pH = 4.68

6. (15 points) The following solutions were "dumped" into a waste container in the laboratory: 50.0 mL of 0.060 M HNO₃, 100.0 mL of 0.010 M HCl and 500 mL of 0.010 M KOH. What is the pH of the waste solution?
pH = 11.19

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