Chemistry 1312
Exam 1, Spring, 2003

I.  Multiple Choice (104 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.     All of the following are conjugate acid‑base pairs except:
a.     H3O+ and OH-
b.     H2S and HS-
c.     C6H5NH3+ and C6H5NH2
d.    HCO3- and CO32-
e.     H3SO4+ and H2SO4

2.     For the reaction, CO(g)  +  3 H2(g)  «  CH4(g)  +  H2O(g), DH° = –206 kJ, what conditions favor an equilibrium shift of reactants to products?
a.     the addition of a catalyst
b.     high pressures and low temperatures
c.     high pressures and high temperatures
d.    low pressures and low temperatures
e.     low pressures and high temperatures

3.     The species in highest concentration in a 0.10 M CH3COOH solution (exclusive of H2O) is
a.     CH3COOH
b.     H3O+
c.     CH3CO2-
d.    OH-
e.     CO2

4.     For the reaction, 4 PCl3(g)  «  P4(g)  +  6 Cl2(g), if initially, [PCl3] = 1.0 mol/L, and if “x” is the equilibrium concentration of P4, the correct mass action expression at equilibrium is:
a.    Kc =  x(6x)6/(1.0-4x)4
b.    Kc  =  6x7/(1.0-4x)4
c.     Kc  =  6x7/(1.0-x)4
d.    Kc = x7/(1.0-x)4
e.     Kc = 6x7

5.     Consider the equilibrium, C(coal)  +  H2(g)  «  CO(g)  +  H2(g).  One way to increase the production of water gas, i.e., CO(g) and H2(g), would be to
a.     increase the coal supply
b.     decrease the water present
c.     add a catalyst|
d.    remove the H2 product
e.     increase the pressure of each of the component gases

6.     Consider the reaction system at equilibrium,
4 NO(g)  +  6 H2O(g)  « 4 NH3(g)  +  5 O2(g)
Which of the following statements is true?
a.     an increase in the partial pressure of NO, increases the partial pressure of H2O
b.     a decrease in the partial pressure of NH3, decreases the partial pressure of O2
c.     an increase in the partial pressure of O2 increases the partial pressure of NO
d.    a decrease in the volume of the reaction system, decreases the partial pressure of O2
e.     a increase in the volume of the reaction the system, decreases Kc

7.     What is the approximate pH of a 0.010 M H2C2O4 (oxalic acid) solution at 25°C?
a.     < 2.00
b.     > 2.00 but < 7.00
c.     7.00
d.    > 7.00 but < 12.00
e.     > 12.00

8.     Identify the weakest acid in aqueous solution.
a.     HNO3
b.     HClO4
c.     HCl
d.    H3PO4
e.     H2SO4

9.     Kw = 2.38 x 10-14 at 37°C.  For a neutral aqueous solution at 37°C which of the following relationships is true?
a.     pH = 6.81, pOH = 7.00
b.     pH = 6.81, pOH = 7.19
c.     pH = 7.00, pOH = 7.00
d.    [H3O+] = 1.54 x 10-7 mol/L, [OH-] = 1.54 x 10-7 mol/L
e.     [H3O+] = 1.54 x 10-7 mol/L, [OH-] = 6.48 x 10-8 mol/L 

10.  The correct equilibrium constant expression for the reaction, 8 H2S(g)  +  4 O2(g)  «  8 H2O(l)  +  S8(s), is
a.     Kc = [H2S]8[O2]4/[H2O]8[S8]
b.     Kc = [H2O]8[S8]/[H2S]8[O2]4
c.     Kc  = 1/[H2S]8[O2]4
d.    Kc  = [H2S][O2]/[H2O][S8]
e.     Kc = [H2O][S8]/[H2S][O2]

11.  An analysis of a 1.00 L reaction system for 2 NO(g)  +  Cl2(g)  «  2 NOCl(g) shows 2.0x10-3 mol NO, 8.00x10–2 mol Cl2 and 4.00 mol NOCl at a temperature in which Kc = 6.0 x 104.  Which of the following statements is true?
a.     the reaction system is at equilibrium
b.     the reaction system is not at equilibrium, the reaction proceeds right to reach equilibrium
c.     the reaction system is not at equilibrium, the reaction proceeds left to reach equilibrium
d.    the moles of NO, Cl2, and NOCl must all decrease to reach equilibrium
e.     the moles of NO, Cl2, and NOCl must all increase to reach equilibrium

12.  Consider the equilibrium system, 2 PbS(s)  +  3 O2(g)  «  2 PbO(s)  +  2 SO2(g)
Which of the following is/are true regarding changes to the equilibrium system?
a.     An increase in the moles of PbS has no effect on the equilibrium.
b.     An increase in the partial pressure of O2, increases the partial pressure of SO2.
c.     An increase in the volume of the reaction vessel, increases the moles of O2.
d.    A decrease in the moles of SO2 increases the moles of PbO
e.     All of the statements are true.

13.  Consider the following weak acids:
HNO2                                                                                    Ka = 4.5 x 10-4
HCOOH                                                                               Ka = 1.7 x 10-4
HCN                                                                                      Ka = 4.9 x 10-10
CH3COOH                                                                           Ka = 1.8 x 10-5
Identify the conjugate base that, when dissolved in water, would produce the highest pH of the solution.
a.     NO2-
b.     HCO2-
c.     CN-
d.    CH3CO2-
e.     not enough data

II. Exercises (61 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.     (15 points)  A total of 16.0 mol SO3 was placed in a 2.00‑L reaction vessel at 600°C.  At equilibrium, the system was analyzed to contain 2.00 mol O2.  Calculate Kc for the reaction at this temperature.
2 SO3(g)  «  2 SO2(g)  +  O2(g)

Ks = 0.111

2.     (15 points)  A reaction mixture of 0.200 mol of COCl2 and 1.00 mol Cl2 is heated in a 2.00 L reaction vessel to a temperature at which Kc = 3.00 for COCl2(g)  «  CO(g)  +  Cl2(g).  Calculate the equilibrium concentrations for COCl2, CO, and Cl2.

[COCl2] = 0.016 mol/L; [CO] = 0.084 mol/L; [Cl2] = 0.584 mol/L

3.     (15 points)  Determine the pH and the percent ionization of a 0.050 M HNO3 solution.  Ka = ¥

[H3O+] = [NO3-] = 0.050 mol/L

pH = 1.30, 100% ionization because HNO3 is a strong acid.

4.     (16 points)  Determine the pH and the percent ionization of a 0.050 M C6H5COOH solution.  Ka = 6.5 x 10-5

[H3O+] = 1.80 x 10-3  mol/L

pH = 2.74, 3.61% ionization.  . C6H5COOH is a weak acid