Problem Set V Chemistry 3100
Winter 2011
Dr. G. Van Biesen
1. Consider the titration of 25.0 mL of 0.0500 M Sn2+ with 0.100 M Fe3+ in 1 M HCl to give
Fe2+ and Sn4+, using Pt and calomel electrodes.
a) Write a balanced titration reaction
b) Write two half-reactions for the indicator electrode
c) Write two Nernst equations for the cell voltage (ESat. Cal = 0.241 V)
d) Calculate E at the following volume of Fe3+: 1.0, 25.0, 30.0 mL
(This is Problem 15.5 in Harris 8th Ed.)
2. Two possible reactions of MnO4- with H2O2 to produce O2 and Mn2+ are:
Scheme 1: MnO4-  Mn2+
H2O2  O2
Scheme 2: MnO4-  Mn2+ + O2
H2O2  H2O
a) Complete all half-reactions (assume acidic solutions) and write net reactions for both
schemes
b) Sodium peroxyborate tetrahydrate, NaBO3 · 4H2O (FM 153.86), produces H2O2 when
dissolved: BO3- + 2 H2O  H2O2 + H2BO3To decide whether Scheme 1 or 2 occurs, 1.023 g NaBO3 · 4H2O was weighed into a
100 mL volumetric flask, 20 mL of 1 M H2SO4 was added and diluted to the mark with
water. 10.00 mL of this solution was titrated with 0.01046 M KMnO4 solution until a
pale pink colour persisted. How many mL of KMnO4 are required for Scheme 1, how
many for Scheme 2? (Scheme 1 stoichiometry was observed)
(This is Problem 15.16 in Harris 8th Ed.)
3. An aqueous glycerol (C3H8O3 - FM = 92.10) solution weighing 100.0 mg was treated with
50.0 mL of 0.0837 M Ce4+ in 4 M HClO4 at 60 ºC for 15 min to oxidize the glycerol to
formic acid (CH2O2). The excess Ce4+ required 12.11 mL of 0.0448 M Fe2+ to reach a
ferroin endpoint. What is the weight percent of glycerol in the unknown?
(This is Problem 15.18 in Harris 8th Ed.)
4. Some people have an allergic reaction to the food preservative sulfite (SO32-). Sulfite in
wine was measured by the following procedure: to 50.0 mL of wine were added 5.00 mL
of a solution containing (0.8043 g KIO3 (FM=214.00) + 5 g KI (FM=166.00)) / 100 mL.
Acidification with 1.0 mL of 6.0 M H2SO4 quantitatively converted IO3- into I3-. The I3reacts with SO32- to generate SO42-, leaving excess I3-, which required 12.86 mL of
0.04818 M Na2S2O3 to reach the end point.
a) Write the reaction that occurs when H2SO4 is added. Is it necessary to weigh out the
5 g of KI accurately? What about the 1.0 mL of H2SO4?
b) Write a balanced reaction btw I3- and sulfite.
Problem Set V Chemistry 3100
Winter 2011
Dr. G. Van Biesen
c) Find the concentration of sulfite in the wine.
(This is Problem 15.30 in Harris 8th Ed.)
5. Li1+yCoO2 is an anode for Li batteries. Cobalt is present as a mixture of Co(III) and Co(II).
Most preparations also contain inert Li salts and moisture. To find the stoichiometry, Co
was measured by atomic absorption, and its average oxidation state was measured by a
potentiometric titration. For the titration, 25.00 mg of solid was dissolved under N2 in 5.000
mL of 0.1000 M Fe2+ (also containing 6 M H2SO4 + 6 M H3PO4). Unreacted Fe2+ required
3.228 mL of 0.01593 M K2Cr2O7 for complete titration.
a) How many mmol of Co3+ are in 25.00 mg of the sample?
b) Atomic absorption found 56.4% by weight Co. What is the average oxidation state
of Co in the sample?
c) Find y in Li1+yCoO2
d) What is the theoretical quotient (wt% Li / wt% Co) in the solid? The observed
quotient was 0.1388 ± 0.0006. Is this consistent with the average Co oxidation
state?
(This is Problem 15.34 in Harris 8th Ed.)