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Wilfrid Laurier University, Waterloo, Ontario
Session: Winter 2009 (SAMPLE TEST)
Course No: CH234
Course Title: Aquatic Chemistry
Professor: Dr. Scott Smith
(not including coverpage)
Length of Test: 50 minutes
Examination Aids Allowed: calculators are allowed
Location:
Name:
ID #:
Number of pages 4 (14 questions)
Answer all questions in the booklet provided. All questions are of equal weight and part marks will be
assigned. Remember to put your name and id no. on your booklet(s).
Official regulations for Laurier exams are given below:
The doors of the examination room will be opened approximately 10 minutes before the start of the examination. Candidates will be permitted to enter the
examination room quietly up to one half hour after the scheduled start of the exam. Candidates arriving late will not be allowed any extra time. Candidates must
not begin the examination or attempt to read the examination questions until instructed to do so.
Candidates once having entered, may not leave the exam room before completing and submitting the exam unless accompanied by a Proctor. Candidates
are not permitted to submit their examination and leave the examination room until 1 hour after the examination has begun, and in no case before their attendance has been taken. In no case may a candidate leave the room temporarily, for any reason, until 30 minutes after the start of the examination. In order that
remaining candidates are not disrupted, candidates must remain seated and may not leave the examination room during the last 15 minutes of the examination session.
At the close of the examination period, candidates must stop writing immediately. The Presiding Officer may seize the papers of candidates who fail to observe
this requirement, and a penalty may be imposed at the discretion of the instructor. Candidates must submit all their work, according to the instructions of the
Presiding Officer, including all materials and a copy of the examination paper with their name and student ID number written on it. Unused examination booklets
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A candidate who leaves before the examination is over must hand in all completed and attempted work, notes made during the exam, and a copy of the
examination paper with their name and student ID number on it.
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outside the examination room. If books, notes etc. cannot be left outside the examination room, they must be put at the front of the examination room in a place
designated by the proctor before a candidate takes a seat. Candidates are advised not to bring valuables to the examination room.
No electronic or communication devices will be allowed in the examination room, including cell phones, blackberries, pagers, etc. Calculators are not allowed
unless specified by the instructor and indicated on the examination paper. Only non-programmable models authorized by the instructor will be allowed. It is the
candidate’s responsibility to ascertain whether the use of calculators is permitted, and, if it is, whether any restrictions are imposed on the types of calculators that
may be brought to the examination.
Translation dictionaries (e.g. English-French) or other dictionaries, (thesaurus, definitions, technical) are not allowed unless specified by the instructor and
indicated on the examination paper. Electronic dictionaries are never allowed.
Except for bottled water, no food or drink is allowed in the examination room. Candidates with health problems that warrant relaxation of this regulation
should provide medical documentation to the presiding officer prior to the beginning of the examination. Such students should restrict themselves to those items
and packaging that will least distract other examinees.
Candidates are expected to write their examinations in an honest and straightforward manner. Where there are reasonable grounds for believing a violation of
exam protocol has occurred, the candidate will be subject to the disciplinary procedures and sanctions according to the University Calendar.
Only currently registered students will be permitted to write the final exam. Examinations conducted at Wilfrid University will be bound by WLU regulations,
regardless of where the candidate is registered.
Approved by Senate (Oct. 27/2003)
CH234
Sample Test
winter 2009
1. (5 points) What is the ANC (generalized alkalinity) for the following ?
(a) A sample of pure water in equilibrium with carbon dioxide gas. At equilibrium the total
dissolved carbon dioxide is 1.05 ×10−4 M and the pH is acidic. Assume there are no species
other then carbonate species.
(b) 0.1 M Na2 SO4
(c) 0.001 M HCl mixed with 0.005 M NaOH
(d) 0.001 M HCl mixed with 0.005 M Na2 CO3
(e) 0.01 M NH4 Cl and 0.01 M NaCl
2. (5 points) What is the pH of a 0.034 M H2 SO4 solutions?
3. (5 points) Two waters are mixed in a volume ratio of 1:1. The alkalinity of the first water is 1.23
mM and the alkalinity of the second water is 2.34 mM. The alkalinity of the mixed water is 1.785
mM. Explain how the alkalinity value of the mixed waters is calculated and why this is a valid
calculation.
4. (5 points) What is the alkalinity of a 3 mM sodium citrate solution? Citric acid is a triprotic acid
H3 L.
5. (5 points) Silicic acid (H4 SiO4 ) results from the weathering of minerals. Silicic acid can be used by
various microorganisms, such as diatoms, to grow their cell walls. The ability of diatoms to utilize
aqueous silicic acid is dependent on pH though. Diatoms can utilize H3 SiO−
4 but not H4 SiO4 or
2−
H2 SiO4 . For silicic acid the pKa values are pKa1 = 9.83 and pKa2 = 13.17. At what pH would
you expect diatoms to grow best?
6. (5 points) List all of the processes given below that increase alkalinity?
photosynthesis
6CO2(g) + 6H2 O → C6 H12 O6 + 6O2(g)
denitrification
+
5CH2 O + 4NO−
3 + 4H → 5CO2(g) + 2N2 + 7H2 O
dissolution of carbonate
CaCO3(s) + 2H+ → Ca2+ + H2 O + CO2(g)
precipation of carbonate
Ca2+ + CO2−
3 → CaCO3(s)
Al-silicate weathering
2KAlSi3 O8(f eldspar) + 2CO2(g) + 11H2 O →
Al2 Si2 O5 (OH)4(kaolinite) + 2K+ + 2HCO−
3 + 4H4 SiO4(aq)
CH234
Sample Test
winter 2009
7. (5 points) To analyze total sulphide in aqueous samples a “purge and trap” method is used. This
method involves acidifying the sample while blowing argon gas through the sample. The outlet for
the gas passes through a very basic sodium hydroxide solution. The sodium hydroxide solution is
analyzed for sulphur using a colorimetric technique. Henry’s law constant for the reaction
H2 S(g) *
) H2 S(aq)
is KH = 10−0.991 . H2 S has two acid dissociations, pKa1=7 and pKa2=13.9. Explain the chemical
basis for this technique.
1 mark bonus What is the analytical advantage if the basic “trap” solution is 10 mL and the
“purge” sample is 500 mL? Be quantitiative.
8. (5 points) Paleolimnology (such as diatom analysis) can be used to determine water chemistry in
the geologic past. If paleolimnology determines that sediments were deposited from water with a
pH of 5.8, assuming that that water was in equilibrium with atmospheric carbon dioxide, what was
the partial pressure of CO2(g) at the time of deposition?
9. (5 points) Cadmium can be complexed by chloride. A maximum of four chlorine atoms can bind to
a central cadmium atom. Five sets of possible step-wise dissassociation constants are given below.
Which one set of logK values is the only one that can be true.
(a) log K1 = −0.05, log K2 = −0.09, log K3 = −0.9, log K4 = −1.32
(b) log K1 = −1.32, log K2 = −0.09, log K3 = −0.9, log K4 = −0.05
(c) log K1 = −1.32, log K2 = −0.90, log K3 = −0.09
(d) log K1 = −0.05, log K2 = −0.09, log K3 = −0.9, log K4 = −1.32, log K5 = −2
(e) log K1 = −1.32, log K2 = −0.90, log K3 = −0.09, log K4 = −0.05
10. (5 points) Benzoic acid, pKa=4.2, can be used to inhibit biological growth. The following data
show the dependence of the minimum toxic concentration on the pH of the solution:
pH
3.5
4.0
4.5
5.0
5.5
6.0
Minimum benzoic acid required (mM)
1.2
1.6
3.0
7.3
21.0
64.0
Interpret the concentration required for growth inhibition in terms of the acid-base chemistry of
benzoic acid.
Page 2 of 4
CH234
Sample Test
winter 2009
2−
3−
11. (5 points) For the diagram below the chemical species (H3 AsO4 , H2 AsO−
4 , HAsO4 , AsO4 ,
HAsO2 , AsO−
2 ) corresponding to the labels are ...
2−
3−
−
(i) VI=H3 AsO4 , V=H2 AsO−
4 , VI=HAsO4 , III=AsO4 , II=HAsO2 , I=AsO2
2−
3−
−
(ii) I=H3 AsO4 , II=H2 AsO−
4 , III=HAsO4 , IV=AsO4 , V=HAsO2 , VI=AsO2
2−
3−
−
(iii) II=H3 AsO4 , III=H2 AsO−
4 , IV=HAsO4 , V=AsO4 , VI=HAsO2 , I=AsO2
2−
3−
−
(iv) IV=H3 AsO4 , III=H2 AsO−
4 , II=HAsO4 , I=AsO4 , VI=HAsO2 , V=AsO2
(v) none of the above
Eh pH diagram for arsenic
12. (5 points) The toxic amount for a given metal at fixed pH and solution chemistry is 3 µM. If 1
mM Ca(NO3 )2 and 1 mM Mg SO4 are added the toxic amount is found to be 12 µM. Qualitatively
explain this observation.
13. (5 points) The over formation constants for zirconium (Zr4+ ) with chloride are 0.92, 1.32, 1.51 and
1.2 for the formation of ZrCl, ZrCl2 , ZrCl3 and ZrCl4 (charges omitted) respectively. If seawater
is 0.56 M total chloride, what are the most significant zirconium species? List the chloride species
from most to least concentrated?
Page 3 of 4
CH234
Sample Test
winter 2009
14. (5 points) From the following diagram, determine the K values for the following reactions. Note:
Fe(OH)3 and Fe(OH)2 are pure solid species.
* Fe3+ + e−
(i) Fe2+ )
(ii) Fe3+ + 3H2 O *
) Fe(OH)3(s) + 3H+
* Fe(OH)2(s) + 2H+
(iii) Fe3+ + 2H2 O + e− )
(iv) Fe(OH)2 + OH− *
) Fe(OH)3 + e−
Fe pe-pH diagram
Page 4 of 4