The Mystery of the Thirteen Test Tubes
84
THE MYSTERY OF THE THIRTEEN TEST TUBES
Purpose
This experiment is designed to give students who do not perform the standard
qualitative analysis scheme a chance to investigate some simple descriptive
chemistry. It avoids the toxicity concerns of mercury and cadmium, and can be
performed in two laboratory periods.
Materials
A uniquely-labeled set of thirteen unknowns in test tubes contain each of the
following solutions:
1M H2 SO4
6M NH3
1M Ba(NO3)2 1M Cu(NO3)2
1M Na2 S
1M K2CrO4
1M NiSO4
1M AgNO3
0.1M SnCl2
1M Fe(NO3)3
1M KCl
1M K2C 2O4
1M KSCN
Apparatus
Test tubes, stirring rods, Bunsen burner, litmus paper or pH paper and nichrome
wire for flame tests.
Safety Precautions
You cannot tell a harmful chemical from a harmless one just by looking. Follow all
normal laboratory practices during this experiment. Avoid skin contact with the
chemicals. Wear your safety glasses at all times.
Hazardous Waste Disposal
All the solutions may be washed, using water, into the Waste Bottles located in the
fumehoods.
Introduction
Thirteen test tubes with thirteen different solutions have been filled and labeled
with a secret code. The actual identity of each solution is a mystery.
You are the chemical detective. You must correctly identify each solution before time
runs out (at the end of 2 laboratory periods).
Now since the Chemical Abstracts Services (CAS) has registered over ten million
compounds, this would seem to be an impossible task. In this case however, you're
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The Mystery of the Thirteen Test Tubes
in luck! All the other compounds have an alibi for the time in question and we know
for certain what the thirteen solutions are.
The Suspects
H2SO4
KSCN
Cu(NO3)2
SnCl2
Ba(NO3)2
NiSO4
NH3
Fe(NO3)3
AgNO3
KCl
K2 C 2 O 4
K2CrO4
Na2 S
All you have to do is match these identities to the code on each test tube. This kind
of process is called qualitative analysis. This means identifying what is present.
The second semester of General Chemistry is entitled Qualitative Analysis at many
colleges because of the focus on this topic in the second semester. In the last 10
years, however, there has been a shift in emphasis away from areas of descriptive
chemistry, i.e., a physical description of what compounds look like.
The other part of Analytical Chemistry is quantitative analysis. After the
identity of a compound is established, the question is how much? Many of your
other experiments were intended to address this question. You will recall that the
answer was often a "number".
You are fortunate to have a limited list of suspects. It would be a much harder job to
identify one out of ten million. On the other hand, your investigation will be severely
limited. No fancy electronic equipment, and you may not question any other
chemicals or have them assist in your investigation. This means that all your
chemical tests will have to make use of these chemicals themselves. Once you
identify one chemical, you may be able to use it to identify others.
You may submit each to a lie detector test in the form of litmus paper or a flame
test.
You must be very careful to avoid contaminating one solution with another. Don't
forget to clean and dry the stirring rod between solutions. Contamination of
solutions is a gross violation of their chemical rights and shall result in dismissed
cases (and possibly no credit).
One set will be provided for each group of detectives. If it is spilled, lost, dropped or
stolen NO MORE will be given. Don't bother consulting your neighbor; every set is
different. Do your own work.
The Mystery of the Thirteen Test Tubes
Before you begin, do some homework. Find out what you can about your suspects.
Make a plan for how you will proceed. Your textbook, The CRC Handbook of
Chemistry and Physics, and other standard reference books might be helpful. The
following may also be useful:
H2SO4
Strong acid, most powerful industrial chemical in the world,
may produce insoluble sulfates if metaled with.
NH3
Ammonia, alias ammonium hydroxide (NH4OH): has done
important work in homes, last known employment as fertilizer,
can turn ugly on any nosy detective.
K2CrO4
Best known for its bright disposition, potassium is almost
inert, but the chromate may drop out if faced with silver, lead,
or barium. Remains bright even when it lays low.
KCl
Nothing but a common salt, almost impossible to recognize in
a crowd, but can be distinguished from other "common salt" by
its pale violet response to any "trial by fire".
Fe(NO3)3
Ferric nitrate: ferric, alias "iron three", is more reactive than
younger brother ferrous; may be recognized by color if not
confused with other species, can be definitively identified by
"bloody" encounter with greatest rival thiocyanate.
K 2 C2 O 4
Potassium oxalate: actions not well known but is of moderate
toxicity, handle with care, believed to have had a "falling out"
with barium.
Na2S
Sodium sulfide, alias "Le Pew": a real loner, possible messy
confrontations with copper, ferric, nickel or tin; tends to linger
on the skin if touched (not recommended).
Cu(NO3)2
Cupric nitrate: first name officially changed to "copper(II)";
leading chemical citizen, many business ventures include
electrical wire manufacturing and production of alloys, notably
brass; in solution easily recognized by "melancholy"
disposition. Once suspected of conspiring with ammonia to
impersonate ink.
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The Mystery of the Thirteen Test Tubes
NiSO4
Nickel(II) sulfate: once very valuable, now net worth greatly reduced;
"Nick" is easily recognized by his "envious" nature.
SnCl2
Stannous chloride, a.k.a. tin(II) chloride: a hard worker, known since
ancient times, currently employed in food packaging industry,
recyclable; fluoride form prevents tooth decay; unfortunate
confrontation with "Le Pew" (Note: the preparation of aqueous
solutions often uses large amounts of HCl which may make the
solution appear more acidic than normal.)
KSCN
Potassium thiocyanate: poisonous little creature, approach with
caution, long-standing "blood" feud with the iron brothers.
Ba(NO3)2
Barium nitrate: little known on this one, chance encounters with
sulfuric acid have often "precipitate" pale consequences.
AgNO3
Silver(I) nitrate: usually pure but can have a "dark side" at times,
has been known to be fatal if taken in by a careless detective.
An anonymous tipster has left this message: "The nitrates are a red herring."
Procedure
In preparation for this experiment, you should prepare a neat, organized data sheet or
table to record your observations on. Bring this table and any notes, books, or lists that
you think you might need to lab.
Round up your container of suspects. Record the code on the bottles for your set on
your report sheet. Take care of your suspects; you cannot get replacements.
You may (and should) perform litmus tests.
Use a nichrome wire to perform flame tests on the suspects. Ask for help if you are
uncertain how to perform or interpret these tests. Take care, however, that you do not
wastefully exhaust all of your solution.
Record any useful observations on your report sheet.
Collect solids and dispose in the trash.
The Mystery of the Thirteen Test Tubes
88
References
Nitz, Otto W., Ondrus, Martin G., and Melton, Tammy.
Laboratory Manual for
Introductory Chemistry, fifth edition: Wm. C. Brown Publishers: Dubuque, 1993, 311 –
315.
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The Mystery of the Thirteen Test Tubes
Mystery of the Thirteen Test Tubes
REPORT SHEET:
Partner Credit:
Name: _______________________
Y
(Circle if yes)
Partner's Name: ________________
Lab Section: _______________
Code identifying your set ____________________
Report the formula of the solution in each bottle and also give the correct chemical name
of each reagent.
1.
___________________________
8. ___________________________
2.
___________________________
9. ___________________________
3.
___________________________
10. ___________________________
4.
___________________________
11. ___________________________
5.
___________________________
12. ___________________________
6.
___________________________
13. ___________________________
7.
___________________________
Report one balanced reaction equation for each of the thirteen compounds identified in this
experiment. The reaction equations reported should be ones that helped you to separate
the compounds from one another (i.e. reactions that showed a positive result). Also
indicate the colour of any precipitate formed in the reactions or the colour of resulting
solutions.
Note: No two reaction equations maybe the same.
Also include neat, organized tables of observations for all reactions and tests performed.
The Mystery of the Thirteen Test Tubes
The Mystery of the Thirteen Test Tubes
Pre-lab Questions:
1.
Be able to name or write the chemical formula for any of the thirteen compounds.
2.
List two of the chemical solutions that have a smell.
3.
Name the two chemical solutions that would turn blue litmus red.
4.
Name the chemical solution that will turn red litmus blue.
5.
Name one major use of the element Cu.
6.
What is the difference between ferrous ion and ferric ion?
7.
Which one of the chemicals may be used in the fertilizer business?
8.
Which chemical solution has had a falling out with barium?
9.
By just looking which of the thirteen chemicals are safe to handle?
10.
Define qualitative analysis.
11.
Define quantitative analysis.
12.
What is involved in a "flame test"?
13.
What is one of the major problems you should try to avoid?
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The Mystery of the Thirteen Test Tubes
EQUILIBRIUM CONSTANTS AT 25°C
Dissociation Constants for Weak Acids & Bases
–
HC2H3O2 <=> H+ + C2H3O2
HCN <=> H+ + CN–
K
1.8 x 10–5
4.0 x 10–10
–
HSO4 <=> H+ + SO 2"
4
1.3 x 10–2
H2S <=> H+ + HS–
1.1 x 10–7
HS– <=> H+!+ S2-
1.3 x 10–13
–
H2CO3 <=> H+ + HCO3
4.2 x 10–7
–
HCO3 <=> H+ + CO 2"
3
4.8 x 10–11
–
HCrO4 <=> H+ + CrO 2"
4
3.2 x 10–7
–
Cr2O 2"
7 + H 2O
! <=> 2HCrO4
+
–
NH4OH <=> NH
! 4 + OH
2.9 x 10–2
1.8 x 10–5
!
Formation Constants for Complex Ions
+
Ag(NH3)2
<=> Ag+ + 2NH3
6 x 10–8
2+ + 4NH
Cu(NH3) 2+
3
4 <=> Cu
5 x 10–15
2+ + 4NH
Zn(NH3) 2+
3
4 <=> Zn
3 x 10–10
2+
2+ + 4NH
Cd(NH
3) 4 <=> Cd
3
!
Ag(CN)2 <=> Ag+ + 2CN–
!
3"
+ + 4CN –
Cu(CN)
4 <=> Cu
!
2+ + 4CN –
Cd(CN) 2"
4 <=> Cd
2+ + 4CN –
Zn(CN) 2"
4 <=> Zn
!
–
–
2+
!CdCl3 <=> Cd + 3Cl
–
+
!HgCl2 (aq) <=> HgCl + Cl
HgCl+ <=> Hg2+ + Cl–
Fe(SCN)2+ <=> Fe3+ + SCN–
9 x 10–8
1.8 x 10–19
2 x 10–27
1.4 x 10–17
1.2 x 10–18
4 x 10–3
3.3 x 10–7
1.8 x 10–7
1 x 10–3
The Mystery of the Thirteen Test Tubes
Solubility Products
AgCl <=> Ag+ + Cl–
1.8 x 10–10
–
Hg2Cl2 <=> Hg 2+
2 + 2Cl
1.4 x 10–18
PbCl 2 <=> Pb2+ + 2Cl–
1.6 x 10–5
2+
+
Ag2 S <=>
! 2Ag + S
10–30
2+
Hg2S <=> Hg 2+
2 + S
10–33
PbS <=> Pb2+ + S2+
10–28
HgS !
<=> Hg2+ + S2+
1 x 10–52
CuS <=> Cu2+ + S2+
1 x 10–37
Bi2 S 3 <=> 2Bi3+ + 3S2+
about
SnS <=> Sn2+ + S2+
SnS2 <=> Sn4+ + 2S2+
10–90
10–26
less than 1 x 10–45
CdS <=> Cd2+ + S2+
1 x 10–28
NiS (freshly ppct.) <=> Ni2+ + S2+
1 x 10–20
NiS (aged) <=> Ni2+ + S2+
about
10–28
ZnS <=> Zn2+ + S2+
1 x 10–20
FeS <=> Fe2+ + S2+
6 x 10–18
MnS <=> Mn2+ + S2+
5 x 10–15
PbSO4 <=> Pb2+ + SO 2"
4
1.8 x 10–8
BaSO4 <=> Ba2+ + SO 2"
4
1.1 x 10–10
2+ + SO 2"
CaSO4 <=> Ca!
4
7.1 x 10–5
PbCrO4 <=> Pb!2+ + CrO 2"
4
10–15
2"
2+
BaCrO4 <=> Ba
! + CrO 4
1.2 x 10–10
2"
CaCrO4 <=> Ca2+
! + CrO 4
7 x 10–4
Ag2CrO4 <=> 2Ag!+ + CrO 2"
4
MgCO3 <=> Mg2+! + CO 2"
3
!
!
1.1 x 10–12
6.8 x 10–6
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The Mystery of the Thirteen Test Tubes
CaCO3 <=> Ca2+ + CO 2"
3
5.0 x 10–9
BaCO3 <=> Ba2+ + CO 2"
3
2.6 x 10–9
2+ + 2OH–
Mg(OH)2 <=> Mg
!
5.6 x 10–12
–
2+
Ca(OH)2 <=> Ca
! + 2OH
4.7 x 10–6
Ba(OH)2 <=> Ba2+ + 2OH–
5.0 x 10–3
Pb(OH)2 <=> Pb2+ + 2OH–
1.4 x 10–20
Cu(OH)2 <=> Cu2+ + 2OH–
1.6 x 10–19
Sn(OH)2 <=> Sn2+ + 2OH–
5.4 x 10–27
Cd(OH)2 <=> Cd2+ + 2OH–
5.3 x 10–15
Fe(OH)2 <=> Fe2+ + 2OH–
4.9 x 10–17
Fe(OH)3 <=> Fe3+ + 3OH–
2.6 x 10–39
Al(OH)3 <=> Al3+ + 3OH–
5 x 10–33
Cr(OH)3 <=> Cr3+ + 3OH–
1 x 10–31
Ni(OH)2 <=> Ni2+ + 2OH–
5.5 x 10–16
Zn(OH)2 <=> Zn2+ + 2OH–
5 x 10–17
Mn(OH)2 <=> Mn2+ + 2OH–
2.1 x 10–13
Mn(OH)3 <=> Mn3+ + 3OH–
1 x 10–36
CuI <=> Cu+ + I–
1.3 x 10–12
PbI2 <=> Pb2+ + 2I–
8.5 x 10–9
HgI2 <=> Hg2+ + 2I–
2.8 x 10–29
AgI <=> Ag+ + I–
8.5 x 10–17