Chemistry 242
Exam 1
February 27, 2013
Name ________________
On my honor, I pledge that I have upheld the Honor Code, and that the work I have done on this assignment has
been honest, and that the work of others in this class has, to the best of my knowledge, been honest as well.
Signed __________________________________________________________________________
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SHOW YOUR WORK. NO WORK, NO CREDIT
(2pts each) Circle the correct answer.
1.
2.
3.
4.
5.
Beer's law states that the relationship between the absorbance of a solution and the concentration of the absorbing species
in a solution is linear. This relationship is most likely to FAIL when
a.
the absorbing species is very dilute.
b.
the absorbing species participates in
concentration-dependent chemical equilibrium.
a
c.
a monochromatic laser is used for the light source.
d.
a mixture of ions are present in the solution
evaluated.
Two methods of sample preparation that are especially useful for gas chromatography are
a.
solid-phase microextraction and purge and trap.
c.
dry ashing and wet ashing.
b.
acid digestion, rotoevaporation
d.
microwave assisted extraction and supercritical fluid
extraction.
When referring to the data obtained from an analysis, the word “precision” indicates
a.
how close the mean of the data set is to the true
value for the composition of the analyte.
c.
how close the sample mass is to the mass specified
in the standard method used.
b.
relative error calculated from the mean of the data
set.
d.
how close the individual data points in the data set
are to each other.
Quantitating an unknown requires comparing to standards. This may be done in one of three ways: use calibration curves;
use standard additions; or use internal standards. The method of standard additions would be used when
a.
a calibration curve is not linear.
c.
the quantity of sample analyzed or the instrument
response varies from run to run.
b.
the standard and analyte have similar
characteristics that are unaffected by the other
material in the sample.
d.
the sample composition is unknown or complex
and affects the analytical signal.
Which of the following is NOT true regarding a blank solution?
a.
Blank solutions contain known concentrations of
analyte.
c.
A blank solution corrects for impurities or
interfering species in the analytical samples.
b.
Blank solutions contain all of the reagents and
solvents used in the analysis with no deliberately
added analyte.
d.
A blank solution is pure solvent with nothing
added.
(rest of exam on the back)
1
On the rest of the exam:
q
q
q
q
INCLUDE LABELS AND PROPER NUMBER OF SIG FIGS
USE PAPER PROVIDED -- ONE SIDE OF PAGE
NAME OR INITIALS ON EACH PAGE.
UP TO 75% OF THE CREDIT GIVEN FOR CORRECT SET UP, INCLUDING LABELS AND SIG FIGS.
6.
(10 pts) A sample in a 1.0 mm cell transmits 75.0% of the incident light at 510 nm. If the solution is 0.075 M, what is its
molar absorptivity?
7.
(10 pts) An analytical procedure requires 50.0 mL of a standard 100.0 ppm chromium solution. How many milligrams of
primary standard potassium dichromate (K2Cr2O7) are required to prepare this solution?
8.
(10 pts) 300.0 mL of an iodide solution was treated with 50.00 mL of 0.3650 M AgNO3. The solid AgI precipitate was
removed by filtration and 1.0 mL of 1% Fe(NO3)3 was added to the filtrate. This solution required 37.60 mL of 0.28790 M
KSCN to reach a light red endpoint. How many milligrams of iodide were there in the original solution? In this titration,
AgSCN precipitates. When all the Ag+ has precipitated, the excess SCN-1 reacts with Fe+3 forming the red complex
[Fe(SCN)]+2 signaling the endpoint of the titration.
When you are finished with this portion of the exam:
!sign the honesty pledge or see the instructor
!attach this page to your answer sheets & turn them in
!pick up the take home exam
2
Chemistry 242
Exam 1 Take Home
Name ________________
(print)
February 27, 2013
Due Monday, March 6, 2013 by 5:00 pm
On my honor, I pledge that I have upheld the Honor Code, and that the work I have done on this assignment has
been honest, and that the work of others in this class has, to the best of my knowledge, been honest as well.
Signed __________________________________________________________________________
If you feel you can’t sign this, contact the instructor (e-mail or in person)
SHOW YOUR WORK. NO WORK, NO CREDIT
The problems are stated as you might encounter them in the "real world." That means that all the steps needed to
arrive at the final answer may not be explicitly stated. Consider all the pertinent aspects of statistical analysis for
each problem.
!
!
!
!
!
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1.
Work on your own, not in groups. Use any books, notes, calculators, and/or computers. You may consult with the
instructor for clarification, hints or help in using the computers, but I may not answer all your questions.
Show your work in a logical sequence so I can follow it. Write down enough steps so I can see what you did and
locate where you may have made errors.
WORK ON ONE SIDE OF PAGE ONLY
NAME OR INITIALS ON EACH PAGE.
INCLUDE LABELS AND PROPER NUMBER OF SIG FIGS
UP TO 75% OF THE CREDIT GIVEN FOR CORRECT SET UP, INCLUDING LABELS AND SIG FIGS.
You don’t have to use a computer for these problems, but if you do:
" Turn in a hard copy of the file including appropriately annotated graph(s) and make sure you show the
formulas as you entered them (document your formulas) so I can follow your calculations.
" Also email the spreadsheet file(s) - include Ex1 and your initials in the spreadsheet file name.
Police have a hit-and-run case and need to identify the brand of red auto paint. The percentage of iron(III) oxide,
which gives paint its red color, found in six samples was: 43.15, 43.81, 45.71, 43.23, 42.99, and 43.56%.
a.
(10 pts) What are the absolute AND relative 95% confidence intervals for this analysis?
b.
(5 pts) The iron from 0.500±0.002 g of this paint is dissolved into 50 mL of con HNO3, quantitatively
transferred to a Class A 100 mL volumetric flask (±0.08 mL tolerance), and diluted to the mark with deionized
water. If the atomic mass of Fe is 55.85 ± 0.05 g/mole and of O is 16.00 ± 0.03 g, what is the molarity of Fe+3
in the diluted sample and its absolute standard deviation? Treat all uncertainties as standard deviations.
2.
A particular assay method for protein in corn requires a 9.35 g sample to achieve a 1% RSD.
a. (5 pts) What sample size is required to achieve a 5% RSD?
b. (10 pts) If you run assays using the sample size calculated, how many sample replicates must you run to have
95% confidence that the "true value" is within 2.5% of the average?
3.
(10 pts) One candy production line was set to produce a mixture of equal sized yellow and blue jelly beans that was
to be 12.5% blue. What sample size (number of beans) must be taken to ensure that the relative standard deviation
for the number of blue beans is less than 5%.
(rest of exam on back)
1
4.
The Gibbs Free Energy (ΔG) is a function of the enthalpy (ΔH) and the entropy (ΔS)
ΔG = ΔH - TΔS
(1)
where T is the absolute temperature (Kelvin). Furthermore ΔG for a reaction is related to the equilibrium constant
for the reaction by
ΔG = -RT ln(K) = -2.303 RT log(K)
(2)
where R is the Ideal Gas Constant, 8.314 J/K·mole. Combining equations 1 and 2 we have:
-2.303 RT log(K) = H - TS
(3)
After rearranging, equation 3 becomes:
- log(K) = pK =
ΔH
ΔS
1
  2.303 R  T  2.303 R
(4)
This form of the equation predicts a linear relationship between pK and (1/T) as long as ΔH is independent of
temperature (a reasonable assumption for relatively small temperature ranges).
Using the data from 0°C to 100°C for the autoionization of water in Table 6-1 (p 129) in your text, Eqn. 4 above,
and your regression template:
a.
(7 pts) Calculate the absolute 95% confidence interval for the ΔH (kJ/mole) of the autoionization reaction of
water.
b.
(7 pts) Calculate the relative 95% confidence interval (express as a percent) for the ΔS (J/K·mole) of that
reaction.
c.
(6 pts) Appropriately working template with graph.
You can do part a and b in the spreadsheet or separately on paper. Clearly show your work.
2
Chemistry 242
Exam 2
April 8, 2013
Name ________________
On my honor, I pledge that I have upheld the Honor Code, and that the work I have done on this assignment has
been honest, and that the work of others in this class has, to the best of my knowledge, been honest as well.
Signed __________________________________________________________________________
If you feel you can’t sign this, contact the instructor (e-mail or in person)
(2pts each) Circle the correct answer.
1.
2.
3.
4.
5.
Which of the following processes is true regarding the digestion of a solution containing a precipitate?
a.
Digestion tends to increase particle size of the
precipitate.
b.
Digestion is used to slow down the
coagulation of a colloidal precipitate because
of the increased energy added to the solution.
c.
A long digestion is not recommended because
the higher temperatures enable impurities to
become trapped in the crystal lattice of the
precipitate.
d.
Digestion ensures complete mixing of the
reagents.
If equal volumes of the following pairs of solutions were mixed, which one would form a buffer?
a.
0.10 M HCl and 0.10 M NaCl
c.
0.10 M NH3 and 0.050 M HCl
b.
0.10 M H2SO4 and 0.10 M NaHSO4
d.
0.10 M HNO2 and 1.0 x 10-4 M NaNO2
Which indicator would work best for the titration of 0.2500 M sodium nitrite (beaker) with 0.1000 M sulfuric acid
(buret)?
a.
Phenolphthalein
c.
Bromcresol green
b.
Bromthymol blue
d.
Phenol red
Which of the following is a reason for “endpoint error” in a titration?
a.
Overtitrating the solution
c.
Misreading the buret
b.
Titrating the indicator
d.
All of the above
Even in the most precise calculations, activity coefficients are never used in which equation?
a.
Water Kw = [H+][OH-]
c.
Insoluble salt Ksp = [M+2][A-]2
b.
Weak acid Ka = [H+][A-]/[HA]
d.
Mass balance 0.25 M = [HA] + [A-]
(remainder of the exam on the back)
On the rest of the exam:
1
SHOW YOUR WORK. NO WORK, NO CREDIT
!
!
!
!
INCLUDE LABELS AND PROPER NUMBER OF SIG FIGS
USE PAPER PROVIDED -- ONE SIDE OF PAGE
NAME OR INITIALS ON EACH PAGE.
UP TO 75% OF THE CREDIT GIVEN FOR CORRECT SET UP, INCLUDING LABELS AND SIG FIGS.
6.
(20 pts) A sample containing 0.2784 grams of sodium chloride (NaCl) and 0.5486 grams of magnesium chloride
(MgCl2). The chloride in the sample was precipitated by the addition of 47.8 mL of a silver nitrate solution. What
is the concentration of the silver nitrate solution?
7.
(25 pts) A solution containing 30.0 mL of 0.1000 M C6H5COOH (benzoic acid) is titrated with 0.2000 M NaOH.
What is the pH of the solution after the addition of 10.0 mL of NaOH? Explicitly state and check any
assumptions you make.
8.
(25 pts) What is the pH of a saturated solution of La(OH)3? Assume the Lewis acid character of the La (III) can
be ignored. Ksp La(OH)3 2 x 10-21.
9.
(20 pts) Set up the system of mathematical equations (not just reactions) needed to solve the problem below without
making assumptions. DO NOT SOLVE, JUST SET UP THE EQUATIONS.
The amino acid glycine (H2NCH2COOH) has two pKa’s, one for the -COOH group (2.35), and one for the -H3N+
group (9.78). What is the pH of 0.025 M sodium glycinate?
Don’t forget the Honesty Pledge
2
Chemistry 242
Exam 3
Name ________________
print
May 8, 2013
On my honor, I pledge that I have upheld the Honor Code, and that the work I have done on this assignment has
been honest, and that the work of others in this class has, to the best of my knowledge, been honest as well.
Signed __________________________________________________________________________
If you feel you can’t sign this, contact the instructor (e-mail or in person)
Table of Constants
J
coul
R  8.314
F  96485.7
Nernst Factor at 25C  0.05916 V
K  mole
mole eAg | AgCl reference  0.197 V SCE reference  0.241V
(2pts each) Circle the correct answer.
1.
Which of the following is the strongest reducing agent under standard conditions?
a.
2.
Sn+4
b.
Sn+2
c.
S2O3-2
d.
H+
The E° for the reduction of phosphate under basic conditions is -1.12 V (reaction below). If the pH is increased,
which statement is true?
PO4-3 + 2 H2O + 1 e- ! HPO3-1 + 3 OH-1
3.
4.
a.
Phosphate will be a stronger oxidizing agent.
c.
Phosphate oxidizing strength will stay the
same.
b.
Can’t predict oxidizing strength from the
information given.
d.
Phosphate will be a weaker oxidizing agent.
Which of the following is not true about the a glass pH electrode?
a.
High concentrations of other positive ions in the solution produce pH readings that are too high.
b.
Highly acidic solutions produce pH readings that are too high.
c.
Varying junction potentials are the main reason the accuracy of pH readings is limited to two decimal places.
d.
The response time for pH electrodes can be quite slow in low ionic strength solutions.
If you are titrating a mixture of tin(II) and iron(II) with a Ce+4 solution, which species will be oxidized first? E° for
Ce+4/Ce+3 = 1.47 V, E° for Fe+3/Fe+2 = 0.771 V, and E° for Sn+4/Sn+2 = 0.15 V.
a.
5.
Fe+2
b.
Sn+2
c.
Ce+4
d.
can’t tell from information given.
Ion selective electrodes that do not involve a redox process produce a voltage that follows the equation given below.
For a cyanide specific electrode which statement is true?
E = const +
when
0.05916
log [X]
n
[CN-]
a.
E decreases
increases.
b.
E increases when [CN-] increases.
c.
d.
The constant decreases when
[CN-] increases.
The constant increases when [CN-] increases.
(remainder of the exam on the back)
On the rest of the exam:
SHOW YOUR WORK. NO WORK, NO CREDIT
!
!
!
!
INCLUDE LABELS AND PROPER NUMBER OF SIG FIGS
USE PAPER PROVIDED -- ONE SIDE OF PAGE
NAME OR INITIALS ON EACH PAGE.
UP TO 75% OF THE CREDIT GIVEN FOR CORRECT SET UP, INCLUDING LABELS AND SIG FIGS.
Table of constants on front side.
6.
Permanganate in neutral or basic solution is often used to identify the presence of alkenes, and with a little
quantitative analysis to also identify the location of the double bond. The unbalance reaction for a particular butene
is:
MnO4-1 (aq) + C4H8 (aq) ! CH3COO-1 (aq) + MnO2 (s)
7.
a.
(20 pts) Balance the reaction with basic conditions.
b.
(10 pts) Write the standard cell (line) notation for this reaction when it is set up as a battery.
c.
(20 pts) If E° for the butene half reaction is +0.35 V, what is Keq for this reaction? (The permanganate E° is
in the table).
(15 pts) Mercaptans (RSH) can be titrated with electrically generated Cu+2 in the reaction
Cu+2 + 2 RSH 6 Cu(SR)2 (s) + 2H+.
Using a copper anode with a current of 0.0121 A it took 120 seconds to completely precipitate all the mercaptan
in 50.0 mL of solution. What was the molarity of the RSH in the solution?
8.
(25 pts) Enough sodium iodide is dissolved in a saturated solution of lead iodide to make the iodide concentration
4.38 x 10-3 M. A strip of lead metal is inserted into the solution to form a half cell. If the Ksp of lead iodide is
7 x 10-9, what is the reduction voltage for this half cell?
Don’t forget the Honor Pledge