Chemistry 1312, Exam 1
Spring, 1997

I. Multiple Choice (90 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. Consider the following system at a dynamic equilibrium:
CH₄(g) + 2 O₂(g) = CO₂(g) + 2 H₂O(l) DH° = –890 kJ
Which of the following stresses on the system does not increase the partial pressure of CO₂?
a. add heat to the system
b. increase the partial pressure of O₂
c. increase the partial pressure of CH₄(g)
d. all increase the partial pressure of CO₂
e. decrease the volume of the reaction vessel

2. All of the following substances dissolved in water are considered “strong” electrolytes except
a. HCl
b. CO₂
c. NaOH
d. KCl
e. H₂SO₄

3. Consider the weak acid solutions, 0.1 M CH₃COOH (K_a = 1.8 x 10^-5) and
0.1 M HCN (K_a = 4.0 x 10^-10) in separate beakers. When analyzing the two beakers, which species (exclusive of water) will have the highest concentration?
a. CH₃COOH
b. CN^-
c. CH₃CO₂^-
d. HCN
e. all have the same concentration

4. Consider the dynamic equilibrium, 2 SO₂(g) + O₂(g) = 2 SO₃(g)
If the initial concentration of SO₂ is [A], the initial concentration of SO₃ is [B], and the equilibrium concentration of O₂ is x, the mass action expression at equilibrium, set equal to Kc, is
a.   d.  equations not available
b.   e.  
c.  

5. The molar concentrations for a reaction system, 2 SO₂(g) + O₂(g) = 2 SO₃(g)
are determined and inserted into the correct mass action expression. The value obtained from the calculation was found to exceed the value of the Kc for the equilibrium system at the temperature of the analysis. What does this observation mean?
a. the system will reach equilibrium at a higher temperature
b. the system will reach equilibrium at a lower temperature
c. more SO₃(g) will exist in the system when equilibrium is achieved
d. more SO₂(g) will exist in the system when equilibrium is achieved
e. the system is in a state of dynamic equilibrium

6. What true statement can be made regarding a 0.0100 M C₆H₅COOH (benzoic acid) solution?
a. the pH of the solution is > 2
b. the pH of the solution is 2
c. the species in highest concentration is C₆H₅CO₂-
d. the pH of the solution is < 2
e. [C₆H₅COOH] = [C₆H₅CO₂-]

7. Four of the following five conditions will favor the formation of nitrogen gas for the endothermic equilibrium, 2 NO(g) + 2 H₂(g) = N₂(g) + 2 H₂O(g)
Which condition does not favor its formation?
a. increasing the heat content of the system
b. increasing the moles of NO(g)
c. reducing the partial pressure of H₂O(g)
d. increasing the moles of H₂O(g)
e. increasing the partial pressure of H₂(g)

8. “pH” has the meaning (for chemists and most other human beings) of
a. (Green Bay) packers Heaven
b. points at the Half
c. pickled Herring
d. patootie Hughey
e. –log [H₃O⁺]

9. Which of the following is not a Brønsted conjugate acid-base pair?
a. H₃O⁺/OH^-
b. F^-/HF
c. CH₃NH₃⁺/CH₃NH₂
d. HCO₃^-/CO₃^2-
e. NH₂^-/NH₃

10. All of the following plots are from data of pH vs. volume of titrant. Which one best represents that for the titration of a weak polyprotic acid with a strong base?
(Plots not available)
a. b. c. d. e.

11. For pure water, K_w = 2.4 x 10-14 at body temperature (37°C). One of the following statements is false about water at this temperature. Which one?
a. [H₃O⁺] = [OH^-]
b. pH + pOH = 13.62
c. pH = 6.81
d. pOH = 7.19
e. [H₃O⁺] = 1.5 x 10^-7 mol/L

12. Which of the following systems will have present the least amount of product(s) at equilibrium conditions?
a. 2 H₂(g) + O₂(g) = 2 H₂O(g) K_c = 1.7 x 10²⁷
b. N₂(g) + O₂(g) = 2 NO(g) K_c = 5.0 x 10^-31
c. H₂(g) + Cl₂(g) = 2 HCl(g) K_c = 3.2 x 10¹⁶
d. 2 HF(g) = Hv(g) + F₂(g) K_c = 1.0 x 10¹³
e. 2 NOCl(g) = 2 NO(g) + Cl₂(g) K_c = 4.7 x 10^-4

13. The conjugate base of H₂PO₄^- is
a. OH^-
b. HPO₄^2-
c. PO₄^3-
d. H₃PO₄
e. H₂O

14. For the reaction, SiO₂(s) + 4 HF(g) = SiF₄(g) + 2 H₂O(g) DH° = –151 kJ
When heat is added to this dynamic equilibrium system
a. K_c increases
b. the concentration of SiO₂ increases
c. the concentration of H₂O increases
d. partial pressure of SiF₄ increases
e. partial pressure of HF increases

15. What is the pH of blood?
a. 1.5
b. 3.98 x 10^-8
c. 6.6
d. 7.0
e. 7.4

II. Exercises. (75 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. (20 points) At 350 K, K_c = 322 for, Br₂(g) + I₂(g) = 2 IBr(g)
A 5.00 L reaction vessel is filled with 3.00 mol Br₂ and 0.400 mol I₂ and heated to 350 K. What are the equilibrium concentrations of Br₂(g), I₂(g), and IBr(g)?

[Br₂] = 0.52 mol/L, [I₂] = about 0 mol/L, [IBr] = 0.160 mol/L

2. (20 points) A mixture of 0.100 mol NO, 0.200 mol H₂, and 0.0800 mol N₂ were placed in a 2.00 L reaction vessel, heated, and allowed to come to equilibrium conditions. At equilibrium, the molar concentration of N₂ was 0.0500 mol/L.
2 NO(g) + 2 H₂(g) = N₂(g) + 2 H₂O(g)

a. (12 pts) What are the equilibrium molar concentrations of NO(g), H₂(g), and H₂O(g)?

[NO] = 0.0300 mol/L, [H₂] = 0.0800 mol/L, [N₂] = 0.0500 mol/L, [H₂O] = 0.0200 mol/L

b. (8 pts) What is the equilibrium constant for the system at these conditions?

K_c = 3.47

3. (35 points) A standard nitric acid solution was prepared by diluting 10.0 mL of 16 M HNO₃ to 5.00 L with deionized water.

a. (5 pts) What is the molar concentration of the (diluted) nitric acid solution?

0.032 mol/L

b. (5 pts) What is the pH of the (diluted) nitric acid solution?

pH = 1.49

c. (15 pts) A 50.0 mL volume of the diluted nitric acid solution is mixed with 7.50 mL of 0.200 M NaOH. What is the pH of the resulting mixture?

pH = 2.76

d. (10 pts) Determine the volume of 0.200 M NaOH required to adjust the solution in c to a pH = 7.

8.00 mL or add another 0.50 mL of 0.200 M NaOH

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