Chemistry 1312
Exam 1, 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. An analysis of the system, PCl3(g) + Cl2(g) « PCl5(g), is 0.060 mol PCl3, 0.18 mol Cl2, and 0.12 mol PCl5 in a 6.0 L reaction vessel. Kc = 11 for the system. Is the reaction at equilibrium? If not in which direction does the reaction proceed to reach equilibrium?
a. Yes, the system is at equilibrium
b. No, the system proceeds right
c. No, but the direction is uncertain
d. No, the reaction proceeds left
e. Unknown, not enough information

2. Consider the endothermic reaction at equilibrium, PCl3(g) + 3 NH3(g) « P(NH2)3(g) + 3 HCl(g). Which of the following changes on the system will increase the amount of HCl(g) when establishing a new equilibrium?
a. Decrease the amount of P(NH2)3
b. Decrease the partial pressure of PCl3
c. Decrease the temperature of the system
d. Increase the volume of the vessel
e. Add a catalyst

3. Kc = 1.84 at 425°C for the equilibrium, H2(g) + I2(g) « 2 HI(g). What is the value of the equilibrium constant for the equilibrium, HI(g) « 1/2 H2(g) + 1/2 I2(g)?
a. 1.84
b. 0.54
c. 0.74
d. 0.92
e. none of the above

4. Methanol is prepared industrially by the reaction, CO(g) + 2 H2(g) « CH3OH(g), DH° = –90.8 kJ Which of the following changes will increase Kc for the reaction?
a. Increase the amount of CO
b. Decrease the temperature of the system
c. Decrease the partial pressure of H2
d. Decrease the amount of CH3OH
e. Decrease the volume of the vessel

5. An analysis of the system, CO(g) + 2 H2(g) « CH3OH(g), at equilibrium is 0.090 mol/L of CO and 0.040 mol/L of CH3OH. Kc for the reaction is 5.0 x 102 at a given temperature. What is the equilibrium concentration of H2?
a. 0.090 mol/L
b. 33.5 mol/L
c. 0.030 mol/L
d. 8.9 x 10-4 mol/L
e. none of the above

6. A reaction mixture of A moles of NO and B moles of H2 is placed into a 1.00 L reaction vessel. The equilibrium constant expression for the system, 2 NO(g) + 2 H2(g) « N2(g) + 2 H2O(g), is which of the following? "x" is the molar concentration of N2(g) at equilibrium.
a. [x][x]2/[A-2x][B-2x]
b. [x][x]2/[A-2x]2[B-2x]2
c. [x][x]/[A-x][B-x]
d. [x][2x]2/[A-2x]2[B-2x]2
e. [x][2x]/[A-2x][B-2x]

7. Formic acid, HCOOH, has a Ka = 1.8 x 10-4. What are the relative molar concentrations of the species (besides water) in a 0.10 M HCOOH solution (at equilibrium)?
a. [H3O+] = [HCO2-] only
b. [HCOOH] > [H3O+] > [HCO2-]
c. [HCOOH] < [H3O+] < [HCO2-]
d. [HCOOH] < [H3O+] = [HCO2-]
e. [HCOOH] > [H3O+] = [HCO2-]

8. An increase in the volume of the reaction vessel (decrease of the external pressure) will result in an increase in the formation of additional product for each of the following reactions systems except
a. 4 NH3(g) + 5 O2(g) « 4 NO(g) + 6 H2O(g)
b. 2 NO(g) + O2(g) « 2 NO2(g)
c. 2 Cu(NO3)2(s) « 2 CuO(s) + 4 NO2(g) + O2(g)
d. 2 NH3(g) + 3 CuO(s) « 3 Cu(s) + 3 H2O(g) + N2(g)
e. 2 NOCl(g) « 2 NO(g) + Cl2(g)

9. The chemical system that reaches equilibrium conditions most rapidly is
a. COCl2(g) « CO(g) + Cl2(g) Kc = 8.05 x 10-4 at 400°C
b. N2(g) + 3 H2(g) « 2 NH3(g) Kc = 0.153 at 450°C
c. CO(g) + H2O(g) « CO2(g) + H2(g) Kc = 0.58 at 1000°C
d. SO2Cl2(g) « SO2(g) + Cl2(g) Kc = 0.045 at 650 K
e. Unknown, not enough information

10. Which of the following solutions has the highest pH?
a. 1.0 M HCl
b. 0.40 M HNO3
c. 0.10 M H2SO4
d. 0.10 M HClO4
e. 0.20 M HF

11. One of the following is not a conjugate acid-base pair. Which one?
a. CH3NH3+/CH3NH2
b. H2SO4/SO42-
c. S2-/HS-
d. H2O/OH-
e. HCO3-/CO32-

12. The pH of a solution is measured at 9.7. What is the [H3O+] in solution?
a. 9.7
b. 5.0 x 10-5
c. 0.99
d. 2.0 x 10-10
e. none of the above

13. The Kw for water at 37°C is 2.38 x 10-14. The following statement is true for water at 37°C:
a. [H3O+] = 1.54 x 10-7, [OH- ] = 6.48 x 10-8
b. pH = 7, pOH = 7
c. pH = 7.19, pOH = 6.81
d. pH = 6.81, pOH = 6.81
e. pH = 6.81, pOH = 7.19

14. Consider the following acids and Ka’s:
CCl3COOH Ka = 0.3
HCN Ka = 4.9 x 10-10
C6H5OH Ka = 1.3 x 10-10
HNO2 Ka = 4.3 x 10-4
HClO2 Ka = 1.0 x 10-2
Which acid has the strongest conjugate base?
a. CCl3COOH
b. HCN
c. C6H5OH
d. HNO2
e. HClO2

15. For an aqueous HCN solution, which of the following changes will increase the pH of the solution?
a. Addition of moles of HCN
b. Addition of moles of CN-
c. Addition of a catalyst
d. Addition of moles of H3O+
e. Removal of moles of H2O

16. For the equilibrium, 4 NH3(g) + 5 O2(g) « 4 NO(g) + 6 H2O(g)
a. Adding NO increases the equilibrium number of moles of H2O
b. Increasing the volume of the vessel increases the equilibrium number of moles of O2
c. Adding NH3 increases the equilibrium constant for the system
d. Removing O2 increases the equilibrium number of moles of NH3
e. Removing H2O increases the equilibrium number of moles of O2

17. For the equilibrium in an aqueous solution, HCO3- + H2O « H3O+ + CO32-, identify the Brønsted acid(s).
a. HCO3- only
b. H3O+ only
c. HCO3- and H3O+ only
d. H2O only
e. HCO3- and CO32- only

II. Exercises (69 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. (22 points) Consider the equilibrium system: CS2(g) + 4 H2(g) « CH4(g) + 2 H2S(g)
A reaction mixture of 0.40 mol CS2 and 1.00 mol H2 is placed into a 400 mL reaction vessel and allowed to react. At equilibrium, 0.20 mol of H2S is found present.
a. (16 pts) What is the equilibrium concentration of each substance in the reaction vessel?

[CS2] = 0.75 mol/L; [H2] =1.5 mol/L, [CH4] = 0.25 mol/L, [H2S] = 0.50 mol/L

b. (6 pts) What is the value of the equilibrium constant for the equilibrium?

Kc = 1.6 x 10-2

2. (12 points) Determine the pH and pOH of a 25.0 mL of 0.0500 M NaOH solution diluted to 250 mL.

[OH-] = 5.0 x 10-3 mol/L, pOH = 2.30; pH = 11.70

3. (20 points) What is the pH and percent ionization of a 0.520 M lactic acid solution (assume a formula of HLac for lactic acid). Ka for lactic acid is 8.4 x 10-4.

pH = 1.68, 4.0% ionization

4. (15 points) Kc = 7.7 x 10-11 at a given temperature for the dissociation of nitrosyl chlroide:
2 NOCl(g) « 2 NO(g) + Cl2(g)
If 0.27 mol NOCl is placed into a 9.00 L reaction vessel at this temperature, what will be the equilibrium concentrations of each substance?

[NOCl] = 0.030 mol/L, [NO] = 5.2 x 10-5 mol/L, [Cl2] = 2.6 x 10-5 mol/L

Return to Chemistry 1312 Exams Page