A) Write the Half-Cell Reaction for the Cathode
Chemistry 331
Exam II
11/30/2006
Take Home
Rules for the Exam: A) The exam must be your own work. B) You may use your personal class notes and any text/reference book you wish. C) The exam is due 12/17/2005 (put the exam under my door) D) Sign you final exam to signify that you followed the rules for the exam.
Assume activity coefficients equal unity unless specifically told to use activity coefficients for a particular problem.
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1) Consider the cell SCE||cell solution|Pt, whose voltage is +0.126 V. The cell solution contains 2.00 mM of Fe(NH4)2(SO4)2, 1.00 mM of FeCl3, 4.00 mM of Na2EDTA, and lots of buffer, pH 6.78, in a volume of 1.00 L.
a) Write the half-cell reaction for the cathode.
b) Find the value of Fe2+/Fe3+ in the cell.
c) Find the quotient of formation constants, Kf(FeEDTA-)/Kf(FeEDTA2-)
2) A sample of Colby Pond water was analyzed for Fe(II) using the FIA system you used in lab. (A colorimetric determination of Fe(II) using the reagent Ferrozine (FZ) in excess.) Given the data in Table 1, calculate the concentration of Fe(II) in the pond water sample and the molar absorptivity of the Fe(FZ)3 complex.
Table 1.
Added Fe(II) (moles/liter) %T
0 (sample) 63.1
1 x 10-6 39.8
2 x 10-6 25.1
3 x 10-6 15.8
Blank (no FZ added) 100
3) (20) Given the following reagents and solutions to work with:
Water
HCl (0.1 M)
NaOH (0.1 M)
Sodium Acetate (Fwt. 82, pK = 4.75)
Ammonium Chloride (Fwt. 53.45, pK = 9.244)
Potassium Hydrogen Phthalate (Fwt. 204, pK1 = 2.95, pK2 = 5.41)
Bromcresol Green (Fwt. = 698, pK = 4.6)
A) Describe how you would prepare a buffer solution with a pH of 4.6, a formal concentration of 0.01 M, and a total volume of one liter. Provide specifics on the quantities of the reagents and solutions that you would use.
B) How many grams of the pH indicator bromcresol green would you need to add to the buffer solution if you wanted the solution to have an absorbance at 600 nm of 0.2. (Assume the pathlength is 1.0 cm, the molar absorptivity at 600 nm for the basic form of bromcresol green is 20,000 and for the acid form it is 0.0 (M-1 cm-1).
4) You are filling in on the set for that famous TV chemist MacGyver. You can save the World from certain destruction if you can make a radio call for help within the next two minutes. Unfortunately, the radio battery is dead! Confident of your chemistry skill, you decide to build your own battery. Using some or all of the following items design your battery.
-assorted lengths of wire
-silver metal
-cadmium metal
-a bottle of 1.0 M AgNO3
-a bottle of 0.1 M CdCl2
-several soda bottles
-paper towel
-water
-high impedance voltmeter
-a pump
-NaCl (s)
-NaNO3(s)
-Na4EDTA (s)
Describe in detail how you would use the above equipment to build a battery. Calculate the potential of the battery.
5) A) A 100 ml sample water sample containing 0.01 M Cu2+ is titrated with a 0.05 M EDTA solution. The pH of the sample has been buffered to pH 9.0 with nonreactive borate buffer. What is the concentration of Cu2+ (report as pCu) after the following volumes of EDTA have been added?
a) 5 ml
b) 10 ml
c) 20 ml
d) 25 ml
Please explain your calculations. Log Kf (CuY2-) = 18.80)
B) How would the your results change if the titration was carried out using 1.0 M ammonia buffer at pH 9.0? Please be specific.