Chemistry 1312, Exam 2

I. Multiple Choice. (96 points) Select the best response for each of the following and record your answers on the Scantron. Do not erase; erasures may result in a misgrading of your response.

1.    All of the following mixtures result in the formation of a buffer solution except
a. Equal volumes of 0.10 M CH₃COOH and 0.10 M NaCH₃CO₂
b. 100 mL of 0.10 M CH₃COOH and 50 mL of 0.10 M NaOH
c. 100 mL of 0.10 M CH₃COOH and 100 mL of 0.10 M NaOH
d. 50 mL of 0.10 M NaHCO₃ and 100 mL of 0.10 M Na₂CO₃
e. 50 mL of 0.10 M NH₃ and 25 mL of 0.10 M HCl

2.    The pH at the stoichiometric point for the titration of a propionic acid solution with a standard sodium hydroxide solution is
a. >7, because of the presence of the sodium hydroxide solution at the stoichiometric point
b. >7, because of the presence of the conjugate base of propionic acid at the stoichiometric point
c. =7, because all acid-base titrations have a pH of 7 at the stoichiometric point
d. <7, because of the presence of the propionic acid at the stoichiometric point
e. dependent upon the molar concentrations of the propionic acid and sodium hydroxide

3.    Which of the following pH curves represents the titration of a nitric acid solution with a standard potassium hydroxide solution? (not shown on this website)
Correct answer: a

4.    For which of the pH curves in Q.3 will the stoichiometric point be less than 7?
a. Curves a. and b.
b. Curve b. only
c. Curve d. only
d. Curves c. and d.
e. Curve e. only

5.    Identify the one salt that has no effect on the pH of an aqueous solution.
a. Ba(NO₃)₂
b. FeCl₃
c. Cr(CH₃CO₂)₃
d. NaHCO₃
e. Na₃PO₄

6.    The mixture of solutions that corresponds to the stoichiometric point in an acid-base titration is
a. 13.0 mL of 0.20 M KOH and 26.0 mL of 0.40 M CH₃COOH
b. 13.0 mL of 0.20 M KOH and 26.0 mL of 0.40 M HNO₃
c. 13.0 mL of 0.20 M KOH and 13.0 mL of 0.40 M CH₃COOH
d. 13.0 mL of 0.20 M KOH and 26.0 mL of 0.10 M CH₃COOH
e. 13.0 mL of 0.20 M KOH and 13.0 mL of 0.10 M HNO₃

7.    A buffer mixture of 2.00 moles of HF and 6.00 moles of NaF is dissolved in 1.00 L of an aqueous solution. What is the result of adding 0.50 moles of hydrochloric acid to the mixture?
a. the pH increases; the moles of HF decreases to 1.50 and the moles of F^- remains unchanged
b. the pH decreases; the moles of HF increases to 2.50 and the moles of F^- decreases to 5.50
c. the pH decreases; the moles of HF decreases to 1.50 and the moles of F^- increases to 6.50
d. the pH remains unchanged; the buffer system resists any changes in pH
e. the pH decreases; the moles of HF increases to 2.50 and the moles of F^- increases to 6.50

8.    The solution having the highest pH is
a. 0.10 M KCN, K_a for HCN = 6.2 x 10^-10
b. 0.10 M Na₃PO₄, K_a3 for H₃PO₄ = 4.8 x 10^-13
c. 0.10 M CH₃COOH, K_a for CH₃COOH = 1.8 x 10^-5
d. 0.10 M Na₂CO₃, K_a2 for H₂CO₃ = 4.7 x 10^-11
e. 0.10 M KF, K_a for HF = 6.3 x 10^-4

9.    A benzoic acid (HBz, K_a = 6.5 x 10^-5) buffer solution with a pH of 4.49 is required. What must be the [Bz^-]/[HBz] ratio in the buffer solution?
a. 4.19 to 1
b. 1 to 1
c. 2 to 1
d. 1 to 2
e. 1.07 to 1

10.    Consider the solution of 0.10 M Na₃PO₄, K_a3 for H₃PO₄ = 4.8 x 10^-13. Of the selections listed, identify the specie(s) that has the highest molar concentration?
a. PO₄^3-
b. H₃O⁺ = OH^-
c. PO₄^3- = OH^- = HPO₄^2-
d. HPO₄^2- = OH^-
e. PO₄^3- = HPO₄^2-

11.    For which of the following reaction mixtures is the K_a = [H₃O⁺]?
a. 10.0 mL of 0.10 M HCOOH + 10.0 mL of 0.10 M NaOH
b. 10.0 mL of 0.10 M HCOOH + 10.0 mL of 0.10 M HCOOK
c. 10.0 mL of 0.10 M HCl + 10.0 mL of 0.10 M NaOH
d. 10.0 mL of 0.10 M HCOOH + 5.0 mL of 0.10 M HCOOK
e. 5.0 mL of 0.10 M HCOOH + 10.0 mL of 0.10 M NaOH

12.    One of the following mixtures results in an acidic solution. Which one?
a. 5.0 mL of 0.10 M HCl + 5.0 mL of 0.20 M KOH
b. 10.0 mL of 0.10 M HCl + 5.0 mL of 0.20 M KOH
c. 10.0 mL of 0.10 M HCl + 15.0 mL of 0.20 M KOH
d. 15.0 mL of 0.10 M HCl + 10.0 mL of 0.20 M KOH
e. 15.0 mL of 0.10 M HCl + 5.0 mL of 0.20 M KOH

13.    All of the following topics were covered in class since Exam 1 except
a. the pH of salt solutions
b. buffer solutions
c. strong acid-strong base mixtures
d. titration of a weak acid with a strong base
e. the fate of the Dallas Cowboys for 2000

II. Exercises. (70 points) Complete the following exercises in the space provided. Your organized and chemically logical (neat) work must be shown; correct answers only account for 10% of the credit indicated.

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

2. (20 points) a. (10 pts) To 25.0 mL of 0.200 M HNO₃ is added 15.0 mL of 0.250 M KOH. What is the pH of the solution?
pH = 1.51

b. (10 pts) To 25.0 mL of 0.200 M HNO₂ is added 15.0 mL of 0.250 M KOH. What is the pH of the solution? K_a of HNO₂ = 5.6 x 10^-4
pH = 3.73

3.    (10 points) Drinking water can be fluoridated with the addition of sodium fluoride. What is the pH of 4,000 L of a 0.15 M NaF solution? K_a for HF is 7.2 x 10^-4
pH = 8.16

4.    (25 points) a. (10 pts) A mixture of 50.0 mL of 0.30 M Na₂HPO₄ and 100 mL of 0.30 M Na₃PO₄ is used to prepare a buffer. What is the pH of the buffer system? K_a3 = 4.8 x 10^-13
pH = 12.62

b. (15 pts) A 0.020 mole sample of HCl was inadvertently added to the solution in Part a. What will be the expected pH and the change in pH of the solution as a result? Assume no volume change.
pH = 11.78; D pH = 0.84

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