Experiment 25
Qualitative Analysis II
GOAL:
This week you develop strategies to use the reactions that you observed in lab last week to identify
unknown solutions. You will also learn to use flowcharts to track your results. While it is important to
learn the reactions, it is far more important to work on the logic required to solve these unknowns.
Especially as the unknowns become more complicated, good logic and thorough record-keeping in
flowcharts will be vital.
INTRODUCTION:
Last week you observed some reactions of seven cations, Na1+, Mg2+, Cr3+, Ni2+, Zn2+, Ag1+, and Pb2+, and
four anions, NO31-, SO42+, Cl1-, and I1-. You noted the formation and colors of precipitates and complex
ions. These observations will allow you to separate and identify unknown mixtures of the ions.
Whenever a precipitate forms, the mixture can be centrifuged and separated into the precipitate and
solution portions. This week you will develop strategies for using last weeks tests to separate and identify
unknown solutions. Prior to lab, you will be working on a computer to let you focus all of your attention
on thinking about the samples rather than manipulating the solutions in the tests. In lab, you will work
with real solutions.
What tests do we have to choose from? Last week you saw:
addition of 1 drop of OH1-,
addition of excess OH1-,
addition of NH3,
addition of Cl1-,
addition of I1-,
sodium flame test, and
nitrate brown ring test.
These tests can be done individually or combined. The order of tests can matter a great deal. Recall the
stability sequences from last week
For Ag+:
Ag2O(s) < AgCl(s) < Ag(NH3)2+(aq) < AgI(s)
For Pb2+: PbCl2(s) < PbSO4(s) < PbI2(s) < Pb(OH)2(s) < Pb(OH)42-(aq) in xs OH1For Zn2+: Zn(OH)2(s) < Zn(NH3)42+(aq) < Zn(OH)42-(aq) in xs OH1For Ni2+: Ni(OH)2(s) < Ni(NH3)42+(aq) < Ni(OH)2(s) in xs OH1For Cr3+: Cr(OH)3(s) < Cr(OH)41-(aq) in xs OH1Remember that you can proceed only to the right in each series. So, for example, if you start with
Ag1+(aq) and add OH1-(aq), you will get Ag2O(s). If you add Cl1-(aq) to this, you can convert it to
AgCl(s). On the other hand, if you add the Cl1- first, adding OH1- later cannot take you back to the left to
Ag2O(s). Since the stability sequences are a little bit different for each cation, they can be used to effect
some tricky separations.
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Simple unknowns may be identified with a single test. More complex mixtures require multiple steps.
You need a good understanding of the reactions and a good record of your tests and results to identify
these unknowns. Flow charts are a useful way to organize this information.
Let’s say that you are given an unknown that contains only one of the cations from last week. The
particular solution you get (Unknown #3) is colorless, so you write
Unknown #3, only 1 cation, colorless
Could be Na1+(aq), Mg2+(aq), Zn2+(aq), Ag1+(aq) or Pb2+(aq)
You have already eliminated Ni2+ and Cr3+ based upon color, so there is no reason to include them in your
flowchart. Now which test do you run? Several good choices are available; let’s say you choose to add
excess OH1-. When you do this, you get a brown precipitate. How do you record this in a flowchart?
Show the precipitate to the left, with a double drop line, and the remaining solution to the right, with a
single drop line. You should record this as
Unknown #3, only 1 cation, colorless
Could be Na1+(aq), Mg2+(aq), Zn2+(aq), Ag1+(aq) or Pb2+(aq)
XS OH1-
Brown ppt, Ag2O
so unknown was Ag1+
Colorless
You’ve identified your unknown in one step. Do you really need to record the color of the solution, or
can you just write nothing under the single drop line? It is probably safer to write “colorless.” That way,
when you come back to this later, your observations will be clear.
Will just adding excess OH1- always work so well? That depends on the unknown. Let’s give you
another clear, colorless unknown that contains only one of the seven cations (Unknown #4). This time
when you add excess OH1- you get no precipitate and a colorless supernatant solution. You would write
Unknown #4, only 1 cation, colorless
Could be Na1+(aq), Mg2+(aq), Zn2+(aq), Ag1+(aq) or Pb2+(aq)
XS OH1-
No ppt
colorless soln
Remember that Ni2+ and Cr3+ are eliminated as possibilities off the top since your unknown is colorless.
What does the lack of precipitate tell you? You must not have Ag1+ because that would have given a
brown precipitate, and you must not have Mg2+ because that would have given a white precipitate. So
your unknown must have been Na1+, Zn2+, or Pb2+. Let’s be careful how we write this, however, because
we will have some complex ions after adding OH1-.
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Unknown #4, only 1 cation, colorless
Could be Na1+(aq), Mg2+(aq), Zn2+(aq), Ag1+(aq) or Pb2+(aq)
XS OH1-
No ppt
unknown
can’t be Ag1+ or Mg2+
colorless soln
Na1+ or Zn(OH)41- or Pb(OH)41-
Why is it important to remember that we now have the complex ions? It will affect the next step we do to
identify this unknown. We need to do another test and we can choose to test either some more of the
original unknown solution, or the solution left after the addition of excess OH1-.
How about trying a sodium flame test now? That is a fine choice, but make sure that you test the original
solution! The OH1- solution we added in our first test is actually NaOH(aq). Once this has been added,
later solutions will test positive for Na1+ even if it wasn’t in our original unknown.
What about adding I1- to see if we get that yellow PbI2 precipitate? This is also a fine choice. Which
solution should we test? Again, we need to test the original solution. Look at the stability sequence for
lead. What happens if you add I1- to Pb(OH)42-? Nothing, because PbI2 if left of Pb(OH)42- in the stability
sequence. OK, let’s say that we take some more of our original sample, we do a sodium flame test and
see no yellow color. We take some more of the original sample, add I1-, and get no precipitate. What do
we know? How do we record this? Just keep adding to the flowchart.
Unknown #4, only 1 cation, colorless
Could be Na1+(aq), Mg2+(aq), Zn2+(aq), Ag1+(aq) or Pb2+(aq)
XS OH1-
No ppt
colorless soln
unknown can’t be Ag1+ or Mg2+
flame test
colorless
Not Na1+
I1-
no ppt
Not Pb2+
colorless
Zn2+ only possible
We ruled out everything but Zn2+, so by process of elimination, Unknown #4 must be Zn2+.
Do you always want to go back and do subsequent tests on the original solution? Definitely not,
especially if the unknown could contain multiple ions. Let’s illustrate this by giving you a more
complicated unknown this time to help illustrate how and why.
Your unknown #5 is a pale green solution known to contain three of the seven cations. What does the
green color tell you? It probably contains Ni2+, but we need to be careful since occasionally Cr3+ can take
on a green color (though usually a darker green). In fact, it’s possible that both colored ions are present,
so we can keep our hunch in mind as we choose tests, but still be open to several possibilities. We can’t
rule out any of the seven cations until we do some tests. Let’s try adding NH3 this time. When we do
this, we get a violet solution and some precipitate that turns out to be white after we centrifuge and
decant.
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Unknown #5, 3 ions, green
Likely Ni2+, could include Cr3+(aq), Na1+(aq), Mg2+(aq), Zn2+(aq), Ag1+(aq) or Pb2+(aq)
NH3
white ppt
violet soln.
We need to think carefully about where each of our possible ions would end up and whether we have
ruled any ions in or out. Ni2+ gives a violet solution with NH3, so we have confirmed its presence. Cr3+
should have formed a green/grey precipitate, so we’ve ruled that out. Na1+ would stay in solution. Zn2+
reacts to form Zn(NH3)42+ and Ag1+ forms Ag(NH3)21+. Both are complex ions, so they will be in the
solution if they are present. The white precipitate could be Mg(OH)2 (s) and/or Pb(OH)2(s). Put all this
information in your flowchart as
Unknown #5, 3 ions, green
Likely Ni , could include Cr3+(aq), Na1+(aq), Mg2+(aq), Zn2+(aq), Ag1+(aq) or Pb2+(aq)
2+
NH3
white ppt
could be Pb(OH)2 or Mg(OH)2
unknown definitely doesn’t have Cr3+
violet soln.
definitely contains Ni(NH3)42+-violet
could contain Na1+, Zn(NH3)42+, Ag(NH3)21+
With the precipitate separate from the violet solution, we can now test these separately since we already
know something about what they may contain. How could we decide if the precipitate contains lead or
magnesium or both? Would adding Cl1- or I1- work? Nope! Check the stability sequence for lead to see
why. We could add excess OH1-. If Pb(OH)2 is present, it will dissolve to form Pb(OH)42-, but
Mg(OH)2(s) would remain unchanged. In lab, we add excess OH1- and the precipitate dissolves except
for the tiniest speck. We definitely had Pb2+. Does this test mean we also had Mg2+? Probably not, since
we should expect about half of the precipitate to remain. For now, we’ll rule out Mg2+, but still record
that lingering speck in case our later results are troublesome.
Unknown #5, 3 ions, green
Likely Ni2+, could include Cr3+(aq), Na1+(aq), Mg2+(aq), Zn2+(aq), Ag1+(aq) or Pb2+(aq)
NH3
white ppt
could be Pb(OH)2 or Mg(OH)2
unknown definitely doesn’t have Cr3+
XS OH1-
no ppt (tiny speck)
too little for Mg(OH)2
violet soln.
definitely contains Ni(NH3)42+-violet
could contain Na1+, Zn(NH3)42+, Ag(NH3)21+
colorless soln
must be Pb(OH)42+
OK, we’ve got Ni2+ and Pb2+ confirmed. We’ve ruled out Cr3+ and Mg2+. How do we decide among the
remaining three ions? Let’s think through the options. We could do a sodium flame test. Should we test
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the original solution or the violet one? Actually it doesn’t matter. The NH3 we added won’t affect that
test. What could we do to tell Zn(NH3)42+ from Ag(NH3)21+? How about adding excess OH1-? Look
carefully at the stability sequences. Nope, that won’t help. Zn(NH3)42+ will react to form Zn(OH)42- but
both are clear and colorless and won’t let us distinguish between zinc and silver. How about adding Cl 1-?
That won’t work either since AgCl is to the left of Ag(NH3)21+ in the stability sequence. Adding I1-? Yes!
If Ag(NH3)21+ is present, it will react to form AgI. Should we go back and add I1- to the original solution
or the violet one? In either case AgI will precipitate if Ag1+ was in the unknown. But wait, we know that
Pb2+ was also in the original unknown and it will precipitate with I1-. We want to use the violet solution
to avoid this complication. In lab when we add I1- to the violet solution, we get a light yellow precipitate,
confirming Ag1+.
Unknown #5, 3 ions, green
Likely Ni2+, could include Cr3+(aq), Na1+(aq), Mg2+(aq), Zn2+(aq), Ag1+(aq) or Pb2+(aq)
NH3
white ppt
could be Pb(OH)2 or Mg(OH)2
unknown definitely doesn’t have Cr3+
XS OH1-
violet soln.
definitely contains Ni(NH3)42+-violet
could contain Na1+, Zn(NH3)42+, Ag(NH3)21+
I1-
no ppt (tiny speck)
colorless soln
too little for Mg(OH)2 must be Pb(OH)42+
cream ppt, AgI
violet soln
Ni(NH3)42+
So our Unknown #5 contained Ni2+, Pb2+, and Ag1+. Did we actually rule out Zn2+ and Na1+? Yes, but
only because we knew our unknown had three ions present. If any number of ions could be present then
we would have needed to do even more tests!
Could we have started work with this unknown in other ways? Sure. Some people might prefer to start
with excess OH1-, others like to start by adding I1-. Some routes give quicker and cleaner identifications
than others, but that can vary with the unknown. Don’t be afraid to try different combinations. Don’t be
afraid to start over if your first attempt leads you to a messy, unhelpful dead end.
The unknowns we have worked through in this Introduction are meant to give you a sense of the logic and
decision-making processes that you should use to identify your own unknowns. Flowcharts are an
important way of showing your logic and tracking your results.
None of the unknowns we have worked required us to identify the anions Cl1-, I1-, SO42-, and NO31-.
When working these solutions, you use the brown ring test and reactions with Ag1+ and Pb2+. When you
need to identify both the cation and the anion in a sample, you can include the tests all on one large
flowchart, or do two separate flowcharts.
General advice for solving unknowns:
1.
Take advantage of what you know about the total number of ions present.
2.
Pay attention to colors of solutions and precipitates to rule in or rule out ions.
3.
Start with a test that will let you learn something specific about the sample.
4.
When a precipitate forms, centrifuge and separate it from the supernatant solution.
5.
Use a flow chart to track what you have done
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6.
7.
8.
Label your samples so that you know which is which. For example, the colorless solution left
after adding I1- might be labeled “I1- soln.” or you might call it “soln B” and then write that
identifying label into your flowchart record.
Never use all of your original sample. Use a small amount for your first test and save the rest in
case you need to start over.
Don’t try to lay out one master plan to apply to all unknowns. You must think and re-evaluate
with each step.
PRE-LAB ASSIGNMENT:
Complete the Pre-Lab section below to turn in at the beginning of lab. The pre-lab is designed to give
you practice using flowcharts to think about the logic involved in determining the identities of the
unknown ions in solution.
The software for the pre-lab experiment is called WinQual. You can either download it to your own PC
or use it on the RC computer network. WinQual will allow you to see reactions using the same chemicals
you had in lab last week and to identify unknown solutions with these reactions.
To download WinQual to your PC:
1. Download WinQual from http://www.chem.wwu.edu/qual
2. Install WinQual in default directory: C:\Program Files\WinQual
3. Run the WinQual shortcut on the desktop
4. Choose Enter Registration Code Now:
Name: Roanoke College
Reg. Code: 30050784F2
To use WinQual in a computer lab, log on to the network and then choose the Start, RC lab software,
Chemistry, then WinQual.
Take a few moments to learn about the software. You will save time and frustration in the long run. See
the appendix at the end of this handout about using WinQual.
Choose the top pop-up Experiment menu. Note that here you can explore reactions or solve several
types of unknowns. Our main use of the software will be to give you practice identifying unknowns.
From the Experiment menu, choose the type of unknown that you need to do. To get acquainted with the
program, choose the “One Positive Ion” option.
When you choose a “One Positive Ion” experiment, you will see a picture of a solid: your
unknown cation with a nitrate anion. You will also see the result of putting this solid into water.
To save this treated sample, click on one of the seven Saved Samples slots in the upper right.
Note that you could click on the Trash Can to discard the treated sample. Make one of the Saved
Samples your Active Sample by clicking on it. [If you have trouble working the program or get
unexpected results, it is often because you have forgotten to click on the sample you want to be
your active sample.]
You can now do a Na1+ flame test, do a NO31- test, or add any of the chemicals used in lab to your
sample. Only complete compounds can be added, so for example, if you want to add XS OH1- to
the active sample, you must click on Na1+, XSOH1-, and then ADD. The User Notes field on the
screen provides much helpful information on using the program.
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Right click on any saved sample to review its history.
When you think you know the identity of the cation in the unknown, go to the Experiment dropdown menu and choose Answer. Note that all of the positive ion unknowns have nitrate as the
anion. When you select the answers for these positive ion unknowns, also select nitrate ion. Be
sure that you print your results to include in your report.
PRE-LAB ASSIGNMENT
In the WinQual software, do One Positive Ion unknowns until you have gotten three of these correct.
Note that the anion is always nitrate in these unknowns. Be sure to print your results for inclusion in your
report. Use your summary table from last week, the three flowcharts you wrote for Question 2 in
Experiment 24, the general advice for solving unknowns on Pages 5and 6 of this handout, and the
information elsewhere in the Introduction to guide your test choices. As you work on each unknown,
keep track of your tests and logic using a flowchart. After printing out your results for an unknown,
handwrite your flowchart directly onto the printout. These flowcharts should be fairly simple, something
like those shown in the Introduction for Unknowns 3 and 4.
In the WinQual software, do Two Positive Ions unknowns until you have gotten two of these correct.
Note that the anion is always nitrate in these unknowns. Be sure to print your results for inclusion in your
report. As you work on each of the unknowns, record what you do and see in flowchart format. After you
get your computer printout for each unknown, copy a clean version of your flowchart directly onto the
printout.
What did you learn about identifying unknowns from solving these One and Two Positive Ion unknowns?
Address this in several well-organized paragraphs. Include a discussion of the merits of starting your
identification attempt with several different tests, e.g. starting by adding excess OH1- vs. adding NH3, or
I1-, etc. Support your statements using your results from doing the unknowns and the flowcharts you
wrote for Question 2 in Experiment 24. Which ions seem easiest or most difficult to identify? Which is
easier, colored or colorless unknowns? Why?
Your Pre-Lab results are due at the beginning of your lab period.
IN-LAB PROCEDURE:
In lab, you will be given a set of 7 unknowns that you must identify using the tests available in the
experiment. These will be done with real solutions, not using WinQual.
You will have TWO unknown solutions that contain ONE unidentified cation. Identify these cations.
(Each solution also contains NO3-, which will not interfere with your tests.)
You will have TWO unknown solutions that contains ONE unidentified anion (as well as Na+). Identify
these anions.
You will have TWO unknown solutions that will contain TWO unidentified cations. Identify these
cations.
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The final unknown will be a solid that contains TWO unidentified cations and TWO unidentified anions.
Identify these four ions. As noted in the General Procedures, you start a solid unknown by adding a small
amount of the solid to some distilled water. Adding 0.1 g of solid to 5 milliliters of water is about right.
You may get a precipitate at this point if two of the ions present react to form one. Consult your Reaction
table to consider what that precipitate may be. Do not attempt to keep adding water to get it to dissolve.
Be sure that you
a. Record all of the unknown numbers!
b. Think before you work. Decide which tests will be helpful and run those. Do NOT run every test
on every unknown. This shows poor understanding of the principles.
c. Use the general procedures listed on Page 9.
d. Use your unknown carefully since it is all you have.
e. Don’t contaminate your unknown.
f. Carefully record everything you do and observe.
g. Label test tubes with a marker so that you don’t mix them up!
h. Record the conclusions that you can draw from each test, both about what might and what cannot
be present based on your observations.
i. Collect all used solutions and leftover unknowns in your waste container. After you have
identified all of your unknowns, discard your wastes in the designated container in the hood.
In all cases, the logic you use as described in your flowcharts is FAR more important than getting
an unknown correctly identified. It will count heavily in your report grade. Do not just guess at
unknowns. See your instructor if you are having trouble with the logic.
RESULTS:
For each unknown you analyzed this week,
 the unknown number
 a clean copy of the flow chart showing the logic you used to reach your conclusion, colors and
formulas for all precipitates and solutions
 your final conclusion about the content of that sample
The results should be recorded neatly in your lab notebook. You will turn in your results before leaving
lab this week or by the date noted by your lab instructor. There will be no additional report to turn in
following lab.
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Appendix 1: General Procedures for Exploring Reactions in Lab
Your unknown solutions will be supplied in test tubes. If your unknown is a liquid, begin any of the tests
below by putting about 10 drops of the solution to be tested in a CLEAN test tube. (After cleaning your
test tubes with soap and brush, rinse with distilled water and allow the excess water to drain.) Never do
tests directly in the original test tube of unknown. If your unknown is a solid, add a small amount of solid
to several milliliters of water (about 0.1 grams in 5 mL of water) to prepare a solution that you can then
test. Record colors of precipitates and solutions throughout.
Test for reaction with OHAdd just one drop of OH- solution to your test solution. Observe for the formation of a precipitate.
Tests for reaction with excess OH- or NH3
Add one drop of OH- or NH3 solution and observe. Now continue adding drops until you have 10 drops
added. Observe as you go to determine if a precipitate initially formed, and if so, whether it redissolves in
excess.
Test for reaction with other ions
Add 5-15 drops, observing as you go.
Sodium flame test
Dip a clean cotton swab into the solution to be tested and then heat the tip of the swab in a Bunsen burner
flame. A very intense yellow flame is observed when Na+ is present. Douse the swab in water to
extinguish the flame and dispose of it in the container provided.
Nitrate brown ring test
Put 20 drops of the solution to be tested into a test tube, and then, carefully and slowly, add 20 drops of
concentrated H2SO4. (The test tube will get hot, and may splatter if the acid is added too quickly.) If
necessary, cool this mixture by allowing cold tap water to run over the outside of the test tube. In a second
test tube, dissolve about 0.1 g FeSO4.7H2O in 1 mL of water. Now, hold the test tube containing the
H2SO4 solution at a 45o angle while you allow 5 drops of the FeSO4 solution to run slowly down the
inside of the tube. Do NOT mix. The aqueous FeSO4 should form a layer above the acid. If nitrate ion
is present, a smoky brown ring will form at the solution interface. This ring may take several minutes to
form, and will disappear eventually. Look at the interface between the two layers from several angles. If
you were doing this test on a real unknown sample, and there were a chance that the sample also
contained I1-, this would interfere with the test. To remove I-, add several drops of saturated Ag2SO4
solution until no more AgI precipitates.
Centrifuging
The centrifuge allows you to separate a solid precipitate from its supernatant liquid. Choose two test tubes
that are as similar as possible. You precipitate and its solution will be in one test tube. Add water to the
second test tube to approximate the amount in the first one. Place your test tubes directly across from each
other in the centrifuge. Close the lid. Centrifuge for 1-2 minutes. Allow the centrifuge to come to a
complete stop before removing the tubes. More tubes can be centrifuged at one time, but you must keep
the centrifuge balanced. If you do more than one sample at a time, use a marker to label them.
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Appendix 2: General Procedures for Exploring Reactions with WinQual
Choose the top pop-up Experiment menu. Note that here you can explore reactions or solve several
types of unknowns. We will use this program to Explore reactions.
From the Experiment menu, choose Explore Reactions. A picture of a test tube with only water will
appear. To begin, click one cation, such as Na+, and one anion, such as OH-. The name of this compound
should appear in the Compound to be added box. Click on Add to add this compound to the test tube.
You should now have another picture of a test tube in the Treated sample window. If you right click on
this window, a box will pop up detailing what species, liquids and solids, are present in this solution. In
our example, Na+ and OH- should be listed under liquid composition. We will use this as our base
solution to test the reaction of OH- with other cations and the reaction of Na+ with other cations. Click in
box 2 in the Saved Samples window. You can right click on this picture as well to see what is present in
the solution. At this point, box 1 should have a solution with only water and box 2 should have a solution
with Na+ and OH-.
We now want to test what happens when we add other compounds to this solution of NaOH(aq). We must
make this the active solution. To do so, click in box 2. The Active Sample (previously just water) will be
replaced with our NaOH(aq) solution. You can always check the composition of a given solution by right
clicking on the picture. We will start by testing OH- with other cations. To the active sample add
Mg(NO3)2. Record your observations and the formula of any solid that forms in the appropriate square of
your table. When you are finished, discard your sample by clicking on the trashcan.
Repeat this test with Ni(NO3)2, Cr(NO3)3, Zn(NO3)2, AgNO3, and Pb(NO3)2. You must discard your
sample between each test. If nothing is observed (the result is a clear, colorless solution and all ions are in
the “liquid composition” column of the sample composition box), write “NR” for no reaction in the
appropriate box of your table.
We want to fill in the rest of the table using a similar procedure. Next, make an active sample of NaOH
using excess OH- (select the XS OH- button for the anion). Test the reaction of this salt solution with the
nitrate salt solutions (Mg(NO3)2, Ni(NO3)2, Cr(NO3)3, Zn(NO3)2, AgNO3, and Pb(NO3)2). Do the same
starting with NaCl, NaI, and NaSO4. Also fill in the column for reactions with NH3.
Finally, perform do a Na+ flame test and a NO3- brown ring test on a sample of NaNO3. Prepare this
solution and make it the active sample. To perform the Na+ test, click on Na+? Record the result. To test
for NO3-, click on NO3-? Record the result.
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