Chem. 101 - Midterm 4
May 14, 2009
Name__________________________________________
All work must be shown on the exam for partial credit. Points will be taken off for
incorrect or no units and for the incorrect number of significant figures. Only a non
graphing calculator is allowed. On short answer problems you must show your work in
order to receive credit for the problem. If your cell phone goes off during the exam
you will have your exam removed from you.
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Page 3
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Page 4
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Page 5
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Page 6
(of 8 possible)
Multiple Choice
(of 30 possible)
Subtotal
1
1a) 2 pts
What is the conjugate base of HCl?
Cl_______________
answer
1b) 2 pts
What is the conjugate base of H2SO4?
HSO4_______________
answer
1c)
2 pts
What is the conjugate acid of NH3?
NH4+
_______________
answer
1d) 2 pts
What is the conjugate acid of NaOH?
H2O
_______________
answer
2)
9 pts
Complete and balance the following reactions:
HBr(aq) + NaOH(aq) Æ NaBr(aq) + H2O(l)
6HI(aq) + 3Al(s) Æ 3H2(g) + 2AlI3(aq)
HCl(aq) + NaHCO3(aq) Æ H2O(aq) + CO2(g) + NaCl(aq)
3)
5 pts
What volume of 0.100 M HCL is required to reach the endpoint in the
complete titration of 25.00 mL of 0.350 M NaOH?
HCl(aq) + NaOH(aq) Æ NaCl(aq) + H2O(l)
n
M=
V
n = MV = (0.350 M )(0.02500 L ) = 0.00875molNaOH
⎛ 1molHCl ⎞
0.00875molNaOH ⎜
⎟ = 0.00875molHCl
⎝ 1molNaOH ⎠
n
M=
V
n 0.00875molHCl
=
= 0.0875 L
v=
0.100 M
M
_______________
answer units
2
4a) 3 pts
What is the pH of a solution with an [H3O+] concentration of 6.7×10-4 M?
pH = − log[ H 3O + ] = − log[6.7 × 10 −4 ] = 3.17 _______________
answer
4b) 3 pts
If the pOH is 5.17 what is the OH- concentration?
pH = − log[OH − ]
5.17 = − log[OH − ]
_______________
OH − = 6.8 × 10 −6 M
answer
4c)
6 pts
The concentration of OH- in a sample of seawater is 5.0×10-6 M. Calculate
the concentration of H3O+ ions, and classify the solution as acidic, neutral,
or basic.
K w = 1.0 × 10 −14 = [ H 3 O + ][OH − ]
[ H 3O + ] =
1.0 × 10 −14 1.0 × 10 −14
=
= 2.0 × 10 −9
−
−6
[OH ]
5.0 × 10
or
pOH = − log[OH − ] = − log[5.0 × 10 −6 ] = 5.30
14 = pH + pOH
pH = 14 − pOH = 14.00 − 5.30 = 8.70
pH = − log[ H 3 O + ]
8.70 = − log[ H 3 O + ]
H 3 O + = 2.0 × 10 −9 M
Basic
_______________
answer units
_______________
answer
5)
5a) 2 pts
5a) 2 pts
What is the equilibrium constant for the following reactions?
P4(s) + 5O2(g) ⇌ P4O10(s)
1
_______________
K eq =
[O2 ]5
answer
+
22Cr2O7 (aq) + H2O(l) ⇌ 2CrO4 (aq) + 2H (aq)
K eq =
[ H + ] 2 [CrO 4 2− ] 2
2−
[Cr2 O7 ]
_______________
answer
3
6)
4 pts
When the reaction 2SO2(g) + O2(g) ⇌ 2SO3(g) is at equilibrium at 800 K
the concentrations of SO2 is 3.0×10-3M, the concentration of O2 is
3.5×10-3 M, and the concentration of SO3 is 5.0×10-2 M. What is the
equilibrium constant?
K eq =
[ SO3 ] 2
[5.0 × 10 −2 M ] 2
=
= 7.9 × 10 4
−3
−3
2
2
[ SO2 ] [O2 ] [3.0 × 10 M ] [3.5 × 10 M ]
_______________
answer
7)
7a) 2 pts
7b) 2 pts
7c)
2 pts
7d) 2 pts
Consider the following reaction at equilibrium
C(s) + H2O(g) ⇌ CO(g) + H2(g)
Predict the effect of:
Adding C to the reaction mixture.
No change. As long as there is some solid it will not change equilibrium to
add of subtract it.
_______________
answer
Condensing H2O and removing it from the reaction mixture.
More reactants form (left)
_______________
answer
Adding CO to the reaction mixture
More reactant form (left)
_______________
answer
Removing H2 from the reaction mixture.
More products form (right)
_______________
answer
4
8)
4 pts
What is the molar solubility of Mg(OH)2? Ksp = 2.06×10-13
Mg(OH)2(aq) ⇌ Mg2+(aq) + 2OH-(aq)
K sp = [ Mg 2+ ][OH − ] 2
[ Mg 2+ ] = S
[OH − ] = 2 S
K sp = S [2 S ] 2 = 4 S 3
S =3
K sp
4
=3
mol
2.06 × 10 −13
= 3.72 × 10 −5
L
4
_______________
answer
units
For each of the following reactions, identify the element being oxidized, the
element being reduced, the oxidizing agent, and the reducing agent.
N2(g) + O2(g) Æ 2NO(g)
9a) 4 pts
N 0 Æ +2 lost eN was oxidized
_______________
N2(g) is reducing agent
answer
O 0 Æ -2 gained e
_______________
O was reduced
answer
_______________
O2(g) is oxidizing agent
answer
_______________
answer
4
pts
2CO(g) + O2(g) Æ 2CO2(g)
9b)
C +2 Æ +4 lost eC was oxidized
_______________
CO(g) is reducing agent
answer
_______________
O 0 Æ -2 gained eO was reduced
answer
O2(g) is oxidizing agent
_______________
answer
_______________
answer
9)
5
Balance the following redox equations:
10)
Mg(s) + Cr3+(aq) Æ Mg2+(aq) + Cr(s)
10a) 4 pts
Mg(s) + Cr3+(aq) Æ Mg2+(aq) + Cr(s)
Mg 0 Æ 2+ lost e- Mg is oxidized
Cr 3+ Æ 0 gained e- Cr is reduced
Oxidation ½ reaction
Mg Æ Mg2+ + 2eReduction ½ reaction
3e- + Cr3+ Æ Cr
3(Mg Æ Mg2+ + 2e-)
3Mg Æ 3Mg2+ + 6e2(3e- + Cr3+ Æ Cr)
6e- + 2Cr3+ Æ 2Cr
Balanced Reactions: 3Mg(s) + 2Cr3+(aq) Æ 3Mg2+(aq) + 2Cr(s)
_______________________________
answer
10b) 4 pts Al(s) + Ag+(aq) Æ Al3+(aq) + Ag(s)
Al(s) + Ag+(aq) Æ Al3+(aq) + Ag(s)
Al 0 Æ 3+ lost e- Al is oxidized
Ag 1+ Æ 0 gained e- Ag is reduced
Oxidation ½ reaction
Al Æ Al3+ + 3eReduction ½ reaction
e- + Ag+ Æ Ag
Al Æ Al3+ + 3eAl Æ Al3+ + 3e+
3(e + Ag Æ Ag)
3e- + 3Ag+ Æ 3Ag
Balanced Reactions: Al(s) + 3Ag+(aq) Æ Al3+(aq) + 3Ag(s)
_______________________________
answer
6
Multiple Choice Questions
CIRCLE THE CORRECT ANSWER
1)
3 pts
The Bronsted-Lowery definition of a base is:
A) A proton donor.
B) A proton acceptor.
C) Produces H+ in solution.
D) Produces OH- in solution.
E) None of the above
Answer: B
2)
3 pts
Consider a 1.6×10-3 M solution of HNO3. Which of the following
statements is NOT true?
A) This solution would turn litmus to red.
B) This solution could neutralize a base.
C) This solution has a pH of 11.20.
D) This solution could dissolve a metal.
E) None of the above
Answer: C
3)
3 pts
Which solution below is considered to have basic character?
A) pOH = 7
B) pH = 2
C) pOH = 4
D) pOH = 13
E) None of the above
Answer: C
4)
3 pts
Which of the following statements are TRUE of buffer solutions?
1. A buffer solution can be made by mixing equal concentration
of acetic acid and sodium acetate.
2. A buffer solution can be made by mixing equal
concentrations of hydrochloric acid and sodium chloride.
3. A buffer solution resists changes in pH when small quantities
of acid or base are added.
A) 1 and 2 only
B) 2 and 3 only
C) 1 and 3
D) All of 1, 2, and 3
E) None of 1, 2, and 3
Answer: C
7
5)
3 pts
Which of the following changes will increase the reaction rate?
A) An increase in the concentration of the products.
B) A decrease of the reaction temperature.
C) Allowing more time for the reaction.
D) An increase in the concentration of reactants.
E) All of the above
Answer: D
6)
3 pts
Which of the following is TRUE for a system that is in dynamic
equilibrium?
A) The forward reaction goes to 100% completion.
B) The reaction rate of the forward reaction approaches zero.
C) The concentration of products is equal to the concentration of the
reactants.
D) Both the forward and reverse reactions come to a halt.
E) None of the above
Answer: B
7)
3 pts
Which of the following is TRUE of a system for which Keq <<1?
A) It will take a long time to reach equilibrium.
B) It will take a short time to reach equilibrium.
C) The equilibrium favors the reverse reaction.
D) The equilibrium favors the forward reaction.
E) None of the above
Answer: C
8)
3 pts
Which compound is the most soluble?
A) Cus, Ksp = 1.27×10-36
B) Pbs Ksp = 9.05×10-29
C) AgCl, Ksp = 1.77 ×10-10
D) AgI, Ksp = 8.51×10-17
E) Not enough information
Answer: C
9)
3 pts
The oxidation number of Cr in Cr2O72- is
A) +2
B) -2
C) +7
D) +6
E) None of the above
Answer: D
8
10) 3 pts
In an electrochemical cell, which of the following statements is FALSE?
A) The anode is the electrode where oxidation occurs.
B) A salt bridge provides electrical contact between the half-cells.
C) The cathode is the electrode where reduction occurs.
D) A spontaneous electrochemical cell is called a galvanic cell.
E) All of the above are true.
Answer: E
9