1
Chemistry 265
Final Exam
2012
Name:
A Trip to the Arctic
The Environment.
[1]
1.
The white bears there are disappearing. One reason is
that they are very soluble in water. Why is this?
[2]
2.
Another problem is global warming. This means the ice doesn’t stay around for the polar
bears to hunt on. Name one gas that is a greenhouse gas and say what concentration of it
you would expect to find in dry clean air.
[2]
3.
If there was no pollution around, what pH would you expect the arctic rain to have?
Why?
[3]
4.
Answer either (a) or (b).
(a)
(b)
How do increasing levels of CO2 in the air affect coral reefs? Write equations.
Calcium carbonate is normally not very soluble and will have little effect on the
pH of water. However, when the water contains large amounts of carbon
dioxide from bacterial respiration, the calcium carbonate readily dissolves.
Write an equation to show how this happens.
2
[3]
5.
Some of the small lakes that remain in the summer are buffered with a
carbonate/bicarbonate buffer.
(a)
Analyses of one of these lakes showed that the water contained 0.057 M
carbonate and 0.035 M bicarbonate. What is the pH of the lake?
pKa1 for carbonic acid = 6.351
pKa2 for carbonic acid = 10.329
pH = Pka2 + log [carbonate]/[bicarbonate] = 10.329 + log (.057/.035) = 10.54
[4]
(b)
One day a cleaning truck dumped 1000L of 0.5 M HCl into the lake. How much
did the pH change? The lake had a volume of 67,500 L.
mole carbonate = 3847.5 mol
mole bicarbonate = 2362.5
add 500 mol acid so have mole bicarbonate = 2362.5 +2 500 mol = 4862.5 mol
mole carbonate = 3847.5 mol – 2500 mL acid = 1347.5 mol
Will still be a buffer. pH = Pka2 + log [carbonate]/[bicarbonate] = 10.329 + log(1347.5/4862.5) = 9.772
Change = 0.77 pH units
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[3]
[4]
6.
There are some salts of various kinds found at one of the dumps outside a small town
there. One of the salts wasPbI2.
It was found that only 1.3 mmol/L of PbI2 was able to dissolve in the water from a spring
nearby. Knowing this, calculate the solubility constant for PbI2 in that H20. Assume all
temperatures are 25oC.
Now, assuming that the activity coefficients for Pb2+ and for I- are 0.87 and 0.96
respectively, calculate the true solubility constant.
4
[4]
7.
DO ONE OF THE FOLLOWING:
(a)
When will the activity of a solution of PbI2 in pure water be approximately equal
to its concentration?
Why is the activity of a solution numerically different from its concentration
when the ionic strength is high? A diagram will help your explanation.
(b)
(c)
If we were trying to dissolve PbI2 in seawater, how would that change the
solubility? Explain your reasoning. There is a lot of NaCl. Probably some iodide.
Discuss what effect each of these would have.
Name two (not Pb) metal ions that are toxic to humans and say how they get
into our environment. Why are they toxic?
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[1]
8.
If we don’t wish to solubilize metal ions to release them into the environment, should we
have an acidic or basic pH?
[2]
9.
One way to treat heavy metal poisoning is with a chelating agent. What would this do?
Give an example of a chelating agent.
[4]
10.
Another salt found at the arctic dump was barium oxalate. The pH in some of the ponds
around the dump is pH 4.0. What will be the solubility of barium oxalate in these ponds?
Ksp for barium oxalate = 1 x 10-6
Ka1 = 5.62 x 10-2 Ka2 = 5.42 x 10-5
Because the pond is acidic, we will assume that some of the oxalate will react with the
acid. Barium would react with base- we are in acidic conditions. So we can ignore this
potential reaction.
BaOx ⇌Ba2+ + Ox2Ox2- + H+ ⇌HOxLet s = solubility = [Ba2+]
Mass balance: [Ba2+] = [Ox2- ]+ [HOx-]
[𝐻 + ][𝑂𝑥 2− ]
𝐾𝑎2
S = [Ox2- ]+
=[Ox2- ](1 +
[Ox2- ] =
[𝐻 + ]
)
𝐾𝑎2
𝑠
[𝐻+ ]
(1 +
)
𝐾𝑎2
Ksp for barium oxalate = 1 x 10-6 =
S2 = 1 x 10-6 (1 + 10-4/5.42 x 10-5)
S = 1.3 x 10-3 M
𝑠2
(1 +
[𝐻+ ]
)
𝐾𝑎2
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[2]
11.
Explain how galvanizing (coating iron with zinc) protects iron from rusting.
Collecting Samples
[2]
12.
What are some things to consider when collecting samples?
[1]
13.
If you have a sample containing particulate matter, what do you need to do before it is
ready to be analyzed?
[1]
14.
Name an acid that is useful for preparing organic samples for analysis. Say what it is
used for.
15.
If you need your sampling standard deviation for your particulate matter to be no bigger
than 5 %, how many particles do you need to sample. Assume there are two
components, equal amounts of each.
[3]
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In the Laboratory
[3]
16.
Answer
one of a,b,or c:
(a)
What does SOP stand for?
What does MSDS stand for?
Name an indicator that can be used in an EDTA titration.
NaOH
HCl
TRIS x
(b)
Draw a diagram showing how an electronic balance works.
(c)
Which of the following can be used as primary standards:
carbonate x
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19.
We need to do titrations to determine the hardness of the water.
[1]
What causes water to be hard?
[1]
Why is hard water a problem?
[2]
[3]
How does an ion exchange resin remove ‘hardness’ from water?
We will titrate a 40.00 mL water sample with 0.0200 M EDTA. We will buffer the
reaction at pH 10. If the equivalence point was determined to be at 7.52 mL, what is the
concentration of calcium ions in the water sample?
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[6]
20.
Answer THREE of the following:
(a) What are the benefits of using a chelating agent for a titration of a metal ion rather
than a simple ligand?
(b) Why is a buffer sometimes necessary when carrying out an EDTA titration of a metal
ion?
(c) Why is an auxiliary complexing agent sometimes necessary necessary when carrying
out an EDTA titration of a metal ion?
(d) What buffer could be used to give us pH 10? (Hint: it will also be good as a
complexing agent)
(e) Give an example of a displacement titration involving EDTA.
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[4]
[3]
[2]
21.
22.
23.
We also titrated the water with 0.05 M HCl to an endpoint with methyl red. For a 45.00
mL water sample, the end point was reached after 13.80 mL. Methyl red undergoes a
transition from red to yellow in the range 4.8-6.0. Assuming the only base present is
carbonate -What was the concentration of carbonate in the water?
For carbonic acid, Ka1 = 4.46 x 10-7
Ka2 = 4.69 x 10-11
There were some problems with eutrophication in one of the ponds. Describe a test for
oxygen demand that could be done. What are possible causes of eutrophication?
We need to carry out some measurements for phosphate using visible absorption. What
reagent could we use?. Why is a reagent necessary? We are thinking about a reagent
that absorbs at 830 nm.
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[6]
24.
Pond water. We were told there were some amino acids in it – released from a
pharmaceutical plant nearby.
Histidine is an amino acid with an extra amine functional group.
pKa1 = 1.6
pKa2 = 5.97
pKa3 =9.28
Explain (show the equations) why histidine can be titrated with both acid and base.
Sketch the titration curve you would obtain for each titration.
Mark the appropriate pKa values on each curve.
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[3]
25.
Choose a reference electrode and explain what the half- reaction is and why it works
well as a reference electrode.
[3]
26.
Ascorbic acid (0.0100 M) was added to 10.0 mL of 0.0200 M Fe3+ in a solution buffered
to 0.30, and the potential was monitored with Pt and saturated Ag/AgCl electrodes.
Dehydroascorbic acid + 2H++ 2e-⇌ascorbic acid + H2O
Eo = 0.390 V
[2]
Write a balanced reaction for the titration reaction.
[9]
Using Eo =0.767 V for the Fe3+/Fe2+ couple, calculate the cell voltage when
5.0 mL, 10 mL and 20 mL of ascorbic acid titrant have been added.
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The trip back
On the plane trip back, you have time to work on some problems.
[12]
Choose 3 of the following:
(a)
What is the solubility of Cobalt hydroxide solubility in water?
Ksp for Co (OH)3 = 1.6 x 10-30
What could you add to help it dissolve a lot better?
(b)
In a solution of 0.01 M cerium and 0.01 M barium to which is added iodate, which will
be precipitated first? Would they be easy to separate in this way? Show calculations to
back up your answers.
Ksp for Ce(IO3)3 = 1.4 x 10-11
Ksp for Ba (IO3)2 = 1.5 x 10-9
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(c)
(d)
Determine the solubility product (Ksp) for silver chloride, given the following standard
reduction potentials:
AgCl(s) + e- ⇌ Ag(s) + Cl-
Eo = 0.222 V
Ag + + e- ⇌ Ag(s
Eo = 0.799 V
Determine the formation constant of the complex Ag(NH3)2+ if the potential of a silver
electrode in a solution that is 0.1 F in AgNO+ and 1 F in NH3 is 0.319 V.
Ag + + e- ⇌ Ag(s
Eo = 0.799 V