CHEM 121L
General Chemistry Laboratory
Revision 1.2
An Analysis of Common Alum
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To learn how to perform a quantitative chemical analysis.
To learn about the chemical and physical properties of substances.
To learn about precipitation reactions.
In this laboratory exercise we will analyze the Common Alum, or Potassium Aluminum Sulfate
Dodecahydrate (KAl(SO4)2•12H2O), we produced during last week’s laboratory exercise to
confirm the chemical and physical properties of the substance are consistent with those of known
samples of Alum. We will conduct one qualitative test and perform two quantitative
measurements on our reaction product. The qualitative test involves testing for the presence of
Sulfate (SO42-). We will then measure the Water content of the hydrated material to confirm the
mass percentage Water in our product is in accord with the chemical formula for Alum. Finally,
we will measure the melting point of the solid. In another laboratory experiment we will carryout a spectroscopic analysis of the Aluminum content of our sample. If these tests produce
results that are consistent with the known properties of Alum, we will assume our sample is in
fact Common Alum.
When presented with any sample, be it a material we have synthesized or one which was
collected or isolated, we may ask; “Of what is this sample composed?” To that end, we first
separate the sample into the pure substances of which it might be composed. There are any
number of methods for performing this task; chromatography, distillation, filtration, etc. Once
separated, each individual substance must be characterized and identified. This requires we
perform a series of physical and chemical analyses on each substance in order to determine its
chemical identity.
In the present case, we will assume we are working with a single substance that does not need to
be separated from other substances. Further, we will limit our testing to a couple of simple
chemical and physical tests. In reality, a more robust series of tests is usually required before
any degree of confidence can be placed in the identification of the sample.
We will be confirming that our synthesis of Common Alum was successful. As noted
previously, Common Alum is a hydrated double salt of Potassium and Aluminum:
KAl(SO4)2•12H2O
In our first test we will dissolve the synthesized compound in Water and treat it with Barium
cation (Ba2+) in the form of aqueous Barium Nitrate. This cation reacts with Sulfate Ion to
produce a visible white precipitate.
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KAl(SO4)2•12H2O(aq)
K+(aq) + Al3+(aq) + 2 SO42-(aq) + 12 H2O
(Eq. 1)
Ba2+(aq) + SO42-(aq)
BaSO4(s)
(Eq. 2)
If this test is positive, it does not mean that the sample is definitely comprised of Sulfate, but it is
consistent with this supposition.
Next, the Waters of hydration will be released by heating the sample to form Anhydrous Alum:
KAl(SO4)2•12H2O(s)
KAl(SO4)2(s) + 12 H2O(g)
(Eq. 3)
As the Water is given off as a vapor, its mass can be determined by measuring the mass lost by
the sample as a result of the heating. Once the mass of the Water in the sample is determined, it
is a relatively easy matter to determine the percentage Water in the sample:
% Water = (Mass Water / Mass Sample) x 100
(Eq. 4)
Finally, the melting point of our solid will be measured and compared with that of known
samples of the Alum. Because Alum is reported to have a fairly low melting point, we can make
this measurement in a hot Water bath.
And, as mentioned above, we will perform additional spectroscopic measurements on our sample
to confirm the mass percentage Aluminum in the sample is indeed correct. This will involve
reacting the Aluminum in the sample with a reagent that will produce colored complex. The
deepness of the color will be proportional to amount of complex present. This will allow us to
determine the amount of Aluminum in the original sample. We will do this in a later laboratory
when we are studying spectroscopy.
The results from this collection of tests should be sufficient to reassure ourselves that we did
indeed synthesize Common Alum during our last laboratory period.
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Pre-Lab Safety Questions
This week, some general questions.
1.
What is the intent of the GHS Safety Data Sheet (SDS)? What are the major sections of the
SDS?
2.
What are the major regions of the NFPA 704 "Fire Diamond"? What are the symbols for
Oxidizer and for a substance the Reacts with Water?
3.
What does the ABC rating for a Fire Extinguisher mean?
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Procedure
The following tests will require about 1.5g of Alum. And, you will need about 1g of material for
the spectroscopic anlysis. Make sure you have synthesized enough material to perform these
tests. Otherwise divide you sample in half and use half the material for each of the tests below.
Be sure and scale the other reagents appropriately.
Obtain ~1.5g of your dried Alum crystals and place them in a mortar. With a pestle, grind them
until they are a fine powder. (This should not take much grinding.)
Analysis for Sulfate in the Sample
We will not only test our Alum sample for the presence of Sulfate, we will also test samples of
Chrome Alum (KCr(SO4)2•12H2O) and Sodium Chloride (NaCl) as well. The first of these
additional tests, on the Chrome Alum, is to see that other Alums also contain Sulfate. The
second, on the Sodium Chloride, is to observe a negative test.
1.
Obtain three medium test tubes. Fill each about halfway with distilled Water.
2.
Add a spatula tip full of each sample to a different test tube. Stir each solution with a stir
rod until dissolution is complete.
3.
Add a few drops of 0.2M Ba2+ to each test tube and observe the results.
Analysis of the Water Content of the Sample
1.
Obtain a crucible and lid. Support your clean crucible and lid on a clay triangle and heat with an
intense flame for 5 minutes.
2.
Allow it to cool to room temperature. Weigh and record the mass of the crucible and lid. Handle
the crucible and lid with the crucible tongs for the rest of the experiment; do not use your fingers.
3.
Repeat the intense heating procedure, allow the crucible to cool and re-weigh the crucible
and cover. Check with your instructor to make sure the weight is sufficiently constant, and
all the Water has been driven out of the crucible.
4.
Add at most 1 g of the Alum crystal to the crucible, replace the lid, and record the
combined mass.
5.
Set the crucible and sample in the clay triangle and carefully set the lid a little off of the
crucible's edge to allow evolved gases to escape. At first, heat the sample slowly using the
light blue portion of the burner’s flame and then gradually intensify the heat. The Water
can violently leave the Alum at this point, if it is heated too strongly. Do not allow the
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crucible to become red-hot as this may cause the anhydrous Alum to decompose. If at any
time you observe white smoke being given off, or smell an acrid odor, discontinue heating
immediately, the Sulfate is being decomposed to SO2.
6.
Heat the sample for about 12 minutes. Then, cover the crucible with the lid and allow it to
cool to room temperature. Find and record the combined mass.
7.
Reheat the sample for 4 minutes, allow it to cool, and then check the combined mass. If the
second mass disagrees by more than 5% of the first mass, repeat Step 6 and 7 until a
constant mass is achieved.
8.
Dispose of the anhydrous Alum in the garbage.
Measuring the Melting Point of the Sample
1.
Set up a Water bath, distilled Water in a 250 mL beaker, on a hot plate. Add a stir bar to
the bath.
2.
Load a melting point capillary with your sample. You should have about a half-inch of
sample in the melting point capillary. Your instructor will demonstrate this procedure.
Attach the capillary to a thermometer by tying it in place with a few pieces of rubberband.
The capillary should be attached such that the sample sits next to the bulb of the
thermometer.
3.
Place the sample in your Water bath and support your thermometer by clamping it in place
using a split rubber stopper. This is as pictured below.
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4.
Slowly heat the Water in the beaker at the rate of no more than ~3oC per minute. (The
slower the heating the sample, the better. This allows the sample to fully equilibrate
thermally with the Water bath.) When the solid melts, note the temperature.
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Data Analysis & Conclusions
1.
Are the results of your test for Sulfate positive? Explain.
2.
Determine the Water content of your Alum sample. Determine the percentage error for this
value.
3.
Calculate the mole ratio of Water to Anhydrous Alum and qualitatively compare it to the
true ratio.
4.
Comment on the sources of error for each determination.
5.
Is your measure melting point consistent with the known melting point for Alum? Explain.
6.
Comment on the quality of your sample of synthesized Common Alum.
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Post Lab Questions
1.
What would happen to an Alum sample's reported percent water if it decomposes yielding a
volatile product other than Water? Explain.
2.
What is the general rule for the solubility of Nitrate salts in Water? Is Barium an exception
to this rule? Why is this important in this laboratory exercise?
4.
Equation 2 is written in Net Ionic form. Write the Total Ionic Equation for this reaction.
What are the Spectator Ions for this reaction?