Chem 321
D. Miller
1.
Name
Answer Key
An electrochemical cell was constructed by placing a KClsaturated Ag/AgCl reference electrode (with salt bridge) and a
platinum wire into a solution containing Fe 2+(aq) and Fe3+(aq) and
connecting the two electrodes with appropriate wiring.
a)
Which electrode is the anode in a galvanic cell (one that
has a spontaneous cell reaction). Explain your choice.
(5)
Ecell is positive for a spontaneous cell redox reaction. The only way to combine the
half-cell reactions associated with the electrodes above and obtain a positive Ecell
is to make the reference electrode the anode (Ag(s) + Cl-(aq) X AgCl(s) + e-,
EE = -0.197 V) and the other electrode (Fe3+(aq) + e- X Fe2+ (aq), EE = 0.771 V)
the cathode.
c)
If the voltage (E) for this cell is 0.363 V, use activities
to determine the Fe2+/Fe3+ ratio in the solution. Assume the
ionic strength has been fixed at 0.050 M and that the
activity coefficient of Fe2+ is twice as large as that for
Fe3+.
(8)
Ecell = 0.363 V = Eox + Ered
since the anode potential is fixed ( Eox = -0.197 V),
Ered = 0.363 V - (-0.197 V) = 0.560 V
cathode: Fe3+(aq) + e-
X Fe2+ (aq)
rearranging gives
and
1.845 x 103
Chem 321
D. Miller
2.
(2)
Name
Answer Key
What is the meaning of the molar absorptivity of a substance?
Molar absorptivity (ε) is a measure of how strongly a substance absorbs light at a specific
wavelength.
A solution containing 1.00 mg of iron as the thiocyanate complex
in 100. mL was found to transmit 70.0% of the incident light
compared to an appropriate blank.
a)
What is the absorbance of the solution at this wavelength?
(2)
0.155
b)
What fraction of light would be transmitted by a solution of
this iron complex that is four times as concentrated?
(3)
If the solution concentration increases by a factor of 4 then the absorbance will
also increase by a factor of four.
A = 4(0.155) = 0.620
24.0%
%T = 0.240 x 100 = 24.0%
The visible absorption spectrum for the iron-phenanthroline
complex Fe(phen)32+ in aqueous solution is shown on the DATA SHEET.
A wavelength around 510 nm was used for the absorbance
measurements in lab. Indicate 2 reasons why this wavelength is
more suitable to use than 550 nm? Be specific.
(6)
1. One of the limitations of Beer’s law is that the linear relationship between absorbance
and concentration holds only if the molar absorptivity does not vary significantly over
the narrow band of wavelengths passing through the monochromator. This
requirement is met at the broad maximum of the absorption band, but not at a
wavelength along the steep sides of the absorption band.
2. Using a wavelength at the absorption peak maximum increases the sensitivity of the
absorbance measurements (larger signal for a given concentration).
How does the absorption spectrum for iron atoms in the gas phase
compare to this iron-phenanthroline absorption spectrum? Explain.
(5)
Iron atoms in the gas phase absorb at a series of discrete wavelengths and have very
narrow absorption peaks because the gas-phase atoms are not associated with the many
closely-spaced vibrational energy levels or energy level distortions in solution that are
found for polyatomic species in solution.
What is the radiation source typically employed in:
(3 ea)
atomic absorption spectrophotometers - hollow cathode lamp
visible light spectrophotometers - tungsten lamp
What type of detector is commonly used in UV/VIS and AA
spectrophotometers?
(3)
photomultiplier tube
Chem 321
D. Miller
3.
Name
Answer Key
Briefly explain how plate height is an indicator of how efficient
a column is in separating components.
(4)
Since
it is a measure of band broadening as a component moves through the
column. A successful separation is achieved if this band broadening is small compared to
the separation of the different component bands. Thus, a smaller value of H indicates a
more efficient column.
On a particular GC column dodecane (C 12H26) has a retention time of
7.12 minutes at an oven temperature of 115EC. How will the
retention time for dodecane change if the column temperature is
set to 90EC? Explain why this change occurs.
(5)
If the column temperature is lowered dodecane will move through the column more slowly
and have a longer retention time. It takes longer for the elution to occur because the
vapor pressure of dodecane is reduced at the lower temperature and dodecane spends
more time in the stationary phase and less time in the mobile gas phase.
Why is an internal standard usually added to samples for
quantitative GC measurements?
(4)
An internal standard is used to compensate for variations in the amount of sample
injected into the gas chromatograph for each run. The volume of sample drawn up and
injected by a typical GC syringe is not very reproducible.
Describe the composition of the stationary phase in our HPLC
experiment.
(3)
The nonpolar stationary phase consists of 18-carbon chain molecules covalently bonded
to the surface of small particles of silica.
Caffeine elutes at 1.30 min for our particular reversed-phase
column and a mobile phase consisting of 55% methanol and 45%
water. How will the retention time for caffeine change if a
mobile phase made of 40% methanol and 60% water is used? Explain
why this change occurs.
(5)
If the mobile phase water content is increased from 45% to 60%, the mobile phase is
becoming more polar and less like the stationary phase. Thus, it cannot compete for the
caffeine as effectively so the caffeine migrates through the column more slowly and its
retention time will increase.
Chem 321
D. Miller
4.
Answer Key
Name
Phosphorus in urine can be determined by treating with
molybdenum(VI) and then reducing the phosphomolybdo complex with
aminoaphtholsulfonic acid to give the characteristic molybdenum
blue color. This absorbs at 690 nm. Assume a patient excretes
1270 mL urine in 24 hours. A 1.00-mL aliquot of the urine was
treated with molybdate reagent and aminoaphtholsulfonic acid and
was diluted to a volume of 50.0 mL. A series of phosphate
standards was similarly treated. The absorbance of the solutions
at 690 nm, measured against a blank, were as follows.
Solution
Absorbance
1.00 ppm P
0.205
2.00 ppm P
0.410
3.00 ppm P
0.615
4.00 ppm P
0.820
urine sample
0.684
Calculate the number of grams of phosphorus excreted by the
patient per day.
(10)
Note that the absorbance increases linearly with concentration (Beer’s law holds).
Therefore,
Using the standard with the closest absorbance reading gives
Cunk = (3.00 ppm)(0.684)/(0.615) = 3.34 ppm P in urine
0.212 g