Chem 220 - Exam III – Spring 2009
Name
Date
SHOW ALL WORK!!!!!!
GOOD LUCK
Points Earned
#1
#2
#3
#4
#5
#6
#7
#8
#9
BONUS
Total Points
1. (8 pts TOTAL)
Cs+ + e- Cs(s) E0 = -3.026 V
Tl3+ + 2e- Tl+ E0 = +1.25 V
Au3+ + 2e- Au+ E0 = +1.41 V
Ni2+ + 2e- Ni (s) E0 = -0.250 V
a. (6 pts)Which anode/cathode pair would you use to make an electrochemical cell that would have the greatest potential to do work? (Assume all concentrations of ionic species to be unity)
b. (2 pts) What is the cell potential for your electrochemical cell (Again, assume all concentrations of ionic species to be unity)
2. (12 pts TOTAL)
Mn2+(aq) + IO4- (aq) MnO4-(aq) + IO3-(aq); ACIDIC solution
a. (4pts)Write the half reaction for the reducing agent?
b. (4 pts)Write the half reaction for the oxidizing agent?
c. (4 pts)Write the complete balanced oxidation-reduction reaction
3. (10 pts) Given that the Ka for HNO2 is 7.08 x 10-4, find the [HNO2]/[NO2-] at pH equal to 5.00.
4. (12 pts TOTAL) Answer the following in relation to the cell below:
1. (4 pts)Write the reaction occurring at the Cd(s) electrode
2. (4 pts)Write the reaction occurring at the Ag(s) electrode
3. (1 pts)How are the electrons flowing (draw on cell below)
4. (1 pts)How is the current flowing (draw on cell below)
5. (2 pts)Assuming all ionic species to be unity, what is the standard reduction potential of the anode at equilibrium?
5. (11 pts) You were asked by your Biochemistry professor to make a buffer with a pH equal to 3.50. You decide that you will dissolve the conjugate base in 2.00L of 2M weak acid. How many moles of conjugate base do you need to dissolve in the weak acid?
Acids to choose from
1. citric acid Ka1 = 7.45 x 10-4
2. hypochlorous acid Ka = 3.0 x 10-8
3. nitrous acid Ka = 7.1 x 10-4
6. (12 pts TOTAL)
a. (10 pts) Calculate the potential of the half reaction of a Zn(s) electrode immersed in a saturated solution of Zn(OH)2 (s)that contains 0.309 M Mg(OH)2(aq).
b. (2 pts)Would Zn(s) be a better cathode or anode electrode in an electrochemical cell that you wanted to use to do work
7. (10 pts TOTAL) Answer the following questions with respect to the electrochemical cell below.
a. (2 pts)What is the chemical nature of the cathode (what is the cathode electrode made out of)?
b. (2 pts) What is the chemical nature of the anode (what is the anode electrode made out of)?
d. (6 pts) If asked, we would say that this cell “does not work” even though the cell potential is positive. Explain CLEARLY with DETAIL why we say that this cell “does not work”.
8. (10 pts) Use the graph below to answer the following questions
a. (2 pts) If you are titrating 20.00 mL of 0.200 M acetic acid with NaOH, what is the molarity of the NaOH that you are using to titrate?
b. (4 pts) What is the pKa of the acid that you are titrating?
c. (4 pts) Prove that the pH at 10mL is equal to approximately 4.
9. (15 pts) How much does the pH of 20.0 mL of a 0.75 M ammonium (NH4+) solution change upon the addition of 10.0 mL of 1.0 M NaOH?
BONUS (10 pts)!!!!
What is the concentration in m/L of the AuCl(aq) solution?
E0 for Ag+/ Ag(s) = +0.799 V
E0 for Cr+3/Cr+2 = -0.42 V
E0 for Zn+2/Zn(s) = -0.763 V
E0 for Al+3/Al(s) = -1.662 V
E0 for Au+(aq)/ Au(s) = +1.69 V
E0 for Au3+(aq)/ Au+(aq) = +1.41 V
Al(OH)3 (s) Ksp = 3.0 x 10-34
Zn(OH)2 (s) Ksp = 3.0 x 10-6
ammonia (NH4+) Ka = 5.69 x 10-10