Chemistry 1312, Exam 1
Spring, 2000
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. Which of the following is not a conjugate acid-base (or base-acid) pair?
c. H2SO4, SO42-
a. PO43-, HPO42-
b. H2O, H3O+
d. HS-, S2-
e. OH-, O2-2. Consider the equilibrium, 2 NO(g) + O2(g) « 2 NO2(g),
DH° = –114 kJ. If A moles of NO(g) and B moles of O2(g) are placed into a one-liter reaction vessel, what will be the form of the equilibrium expression for determining the equilibrium molar concentration of NO2(g)? Assume x to be the change in the moles of O2(g) to reach equilibrium.
a. Kc = 2x/(A-2x)(B-x)
b. Kc = (2x)2/(A-2x)2(B-x)
c. Kc = (2x)2/(A)(B)
d. Kc = 2x2/(A-2x)2(B-x)
e. Kc = x2/(A-x)2(B-x)3. Consider the equilibrium, 2 NO(g) + O2(g) « 2 NO2(g),
DH° = –114 kJ. Which of the following changes will increase the equilibrium concentration of O2(g)?
a. A decrease in volume of the reactor
b. A decrease in the concentration of NO(g)
c. A decrease in the temperature of the reaction system
d. A decrease in the concentration of NO2(g)
e. All changes decrease the concentration of O2(g)4. Consider the equilibrium, 2 BrCl(g) « Br2(g) + Cl2(g), for which Kc = 32 at 500 K. An analysis of the 6.00 L reaction system revealed the presence of 0.22 mol BrCl(g), 1.66 mol Br2(g) and 4.03 mol Cl2(g). What can be surmised about the reaction system?
c. the reaction system is not at equilibrium, the reaction must proceed left to reach equilibrium
a. the reaction system is at equilibrium
b. the reaction system is not at equilibrium, the reaction must proceed right to reach equilibrium
d. the equilibrium constant will increase with increasing temperature
e. the molar concentration for all substances will be 32 mol/L at equilibrium5. Consider the equilibrium, 2 BrCl(g) « Br2(g) + Cl2(g), for which Kc = 32 at 500 K. What is the equilibrium constant for the formation of one mole of BrCl(g) from the reaction of Br2(g) and Cl2(g)?
e. 0.18
a. 32
b. 16
c. 9.8 x 10-4
d. 0.031
6. Consider the equilibrium, Ni(s) + 4 CO(g) « Ni(CO)4(g). All of the following statements are true about the equilibrium except
a. Kc = [Ni(CO)4]/[Ni][CO]4
b. An increase in the amount of CO causes the equilibrium to shift right
c. An increase of the external pressure on the reaction system (decrease volume) increases the partial pressure of Ni(CO)4 in the system
d. The removal of Ni(CO)4 from the equilibrium decreases the amount of CO in the system
e. The addition of a gaseous catalyst has no effect of the amount of CO in the system.7. Assuming all of the following acid solutions have the same concentration, which one has the highest pH?
c. HCN, Ka = 4.9 x 10-10
a. HBrO, Ka = 2.0 x 10-9
b. HClO2, Ka = 1.0 x 10-2
d. HNO3
e. HNO2, Ka = 4.3 x 10-48. The Kw for water at 37°C (body temperature) is 2.5 x 10-14. Which of the following statements is true for water at 37°C?
c. pH = 6.80; pOH = 6.80
a. [H3O+] = 1.6 x 10-7 mol/L; [OH-] = 6.3 x 10-8 mol/L
b. pH = 6.80; pOH = 7.20
d. pH + pOH = 14.0
e. [H3O+] = [OH-] = 2.5 x 10-14 mol/L9. Consider the equilibrium, HQ(aq) + H2O(l) « H3O+(aq) + Q-(aq). For this equilibrium Ka is <<1. Which of the following descriptions of the system is correct?
a. HQ is a weaker proton donor than is H3O+
b. HQ and H2O is a conjugate acid-base pair
c. HQ and Q- are both proton donors
d. H2O is a stronger base than is Q-
e. the equilibrium lies far to the right10. Which of the following is the strongest Brønsted base?
d. ethylamine, Kb = 6.5 x 10-4
a. pyridine, Kb = 1.8 x 10-9
b. ammonia, Kb = 1.8 x 10-5
c. morphine, Kb = 1.6 x 10-6
e. nicotine, Kb = 1.0 x 10-611. Kc = 1.6 x 106 for the equilibrium, 2 CO(g) + O2(g) « 2 CO2(g). It can be stated that the reaction
e. goes essentially to completion
a. takes place very rapidly
b. does not take place to any extent
c. does not form an equilibrium
d. only proceeds in one direction
12. Kc = 1.6 x 106 for the equilibrium, 2 CO(g) + O2(g) « 2 CO2(g). If equimolar concentrations of CO(g) and O2(g) are initially placed in a one-liter reaction vessel, what condition will be true at equilibrium?
b. [CO] < [O2]
a. [CO] = [O2]
c. [CO] > [CO2]
d. [CO] = [CO2]
e. [CO2] = [O2]13. For the Brønsted equilibrium of hydrocyanic acid, HCN(aq) + H2O(l) « H3O+(aq) + CN-(aq). which specie(s), besides water, has the highest concentration?
a. HCN
b. H3O+
c. CN-
d. HCN = CN-
e. H3O+ = CN-14. For the equilibrium, CS2(g) + 3 Cl2(g) « S
2Cl2(g) + CCl4(g), ?H° = –84.3 kJ. To shift the equilibrium position to the left,
a. increase the temperature
b. decrease the size of the reaction vessel
c. remove some CCl4 from the system
d. add Cl2 to the system
e. add a negative catalyst15. For aqueous solutions at any temperature, which of the following must always be true for a neutral solution?
b. [H3O+] = [OH-]
a. pH = pOH = 7.0
c. all acids must have the same concentrations as their conjugate bases
d. pH + pOH = 14
e. [OH-] = 1.0 x 10-7 mol/L16. An aqueous solution has a pH = 3.85. Therefore,
a. [H3O+] = 1.4 x 10-4 mol/L
b. [OH-] = 1.4 x 10-4 mol/L
c. [H3O+] = 1.4 x 10-3 mol/L
d. [H3O+] = 6.9 x 103 mol/L
e. [OH-] = 6.9 x 103 mol/L17. For the Brønsted equilibrium for hydrocyanic acid, HCN(aq) + H2O(l) « H3O+(aq) + CN-(aq). what happens to the pH of the solution when solid KCN is added to the system?
b. pH increases
a. pH remains unchanged
c. pH decreases
d. need Ka for HCN
e. pH adjusts to 7.0
II. Exercises. (69 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. (4 points) Write the Brønsted equation that accounts for gaseous hydrogen bromide being an acid in aqueous solution.
HBr(g) + H2O(l) « H3O+(aq) + Br-(aq)
2. (25 points) a. (5 pts) What is the predicted pH of 200 mL of a 0.0340 M HBr solution?
pH = 1.47
b. (10 pts) What is the predicted pH of the solution in 2a after it has been diluted to 500 mL with water?
pH = 1.87
c. (10 pts) What is the predicted pH of 200 mL of a 0.0340 M HBrO solution? Ka = 2.0 x 10-9
pH = 5.083. (20 points) A 0.0340 M HJa solution (hydrojavelinic acid) is determined to be 5.0% ionized.
pH = 2.77
a. (10 pts) What is the pH of the solution?
b. (10 pts) What is the Ka for HJa?
Ka = 8.9 x 10-54. (20 points) Consider the equilibrium, 2 NOCl(g) « 2 NO(g) + Cl2(g). A 0.60 mole sample of NOCl is sealed into a 500 mL vessel and warmed to 35°C where Kc = 1.6 x 10-5. What is the equilibrium concentration of each of the substances at 35°C?
[NO] = 0.036 mol/L; [Cl2] = 0.018 mol/L; [NOCl] = 1.16 mol/L Return to the top of the page Return to Chemistry 1312 Exams Page