Chemistry
October 16, 2013
The Creation and Dehydration of Alum Crystals
Abstract:
The goals of these labs are to create alum crystals and to dehydrate the crystals. This can be
accomplished by performing a series of methods using different equipment for each lab. The results
were large crystals that were then dehydrated causing the crystal to lose mass and the crucible to gain
mass. These results support my hypothesizes that I had stated before doing the lab. I made a few minor
errors while doing this lab and they may have caused my data to be skewed. In this lab, I learned how to
make crystals by using aluminum and potassium hydroxide and mixing that with sulfuric acid. I also
learned how to dehydrate a crystal by removing the extra hydrates.
Introduction:
The learning context for this lab is creating alum crystals from aluminum and hydroxide. Stoichiometry
and mole to mole ratio are two concepts that are needed for this lab. The goals of these labs are to
perform a series of chemical reactions which lead to the synthesis of the ionic compound potassium
alum and to dehydrate the crystals to determine the amount of water in them. My hypothesis for the
creating of the potassium alum crystal was: Al+C+KOH -> C(p)+H2O+AlK. My hypothesis stated that the
carbon from the aluminum foil would precipitate out of the solution. It also stated that the aluminum
would form crystals when put potassium when put in sulfuric acid. This can be shown as:
6AlK+2H2SO4+O2 ->KAl(SO4)2•12H2O. My hypothesis for the dehydration was KAl(SO4)2•12H2O heated
will separate the hydrate from the crystal to make KAl(SO4)2+12H2O.
Materials and Methods:
Materials needed for creating the crystals:
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1 gram aluminum metal
3 Molar potassium hydroxide solution
35 mL of sulfuric acid
50% ethanol water
Glass beakers
Ring stand
Analytical balance
Funnel
Funnel paper
Materials needed for the dehydration of the crystals
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Crucible and cover
Analytical balance
Bunsen burner
Alum crystals
Chemistry
October 16, 2013
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Ring stand
Ring
Clay triangle
Forceps
Crucible tongs
Wire gauze
Method for the synthesis of the crystals
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Mass one gram of aluminum
put the aluminum in a 250 mL beaker
add 25 mL of 3M potassium hydroxide and stir
after all is dissolved, filter into another beaker
rinse the filter paper with deionized water
allow time for solution to cool
after cooled, add 35 mL of sulfuric acid slowly while stirring
filter solution and discard any solids
leave solution over night
pour out the solution and remove crystals
wash crystals with 50% ethanol water
Take mass of crystals
Safety concerns for the synthesis of the crystals
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Use gloves and goggles because the chemicals used are corrosive
Use fume hood because gases are formed during this lab
Handle all chemicals with care
Method for the dehydration of the crystals
1. Heat empty crucible and cover over Bunsen burner for about 5 minutes to get any absorbed
water out
2. Remove from burner and place on wire gauze. Allow adequate time to cool
3. Mass the empty crucible and crystal
4. Mass 2 grams of crushed alum crystals in the crucible
5. Set up ring, ring stand and clay triangle over a Bunsen burner. Set the crucible with the crystals
on the clay triangle. Put the cover on loosely leaving it a little off.
6. Heat crucible for 5 minutes.
7. Mass the crucible and the dehydrated crystal
Safety concerns for dehydrating the crystals
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Do not touch the crystals because they are rinsed with 50% ethanol water
Be cautious while using the Bunsen burner and always keep someone present with it while on
Chemistry
October 16, 2013
Results:
Table 1: Raw data for synthesis of crystals
Mass of aluminum
Mass of crystals formed
0.9995 g
11.0870 g
Table 2: Raw data for dehydration of crystals
Mass of crystals before heating
Mass of crystals after heating
Mass of crucible before heating
Mass of crucible after heating
2.0002 g
1.0685 g
23.5758 g
23.579 g
Graph 1: Mass of Crystal and Crucible Before and
After Heating
25
23.579
23.5758
20
15
weight in grams
10
5
0
2.0002
crystals before
heating
1.0685
crystals after
heating
crucible before
heating
crucible after
heating
Table 3: The Change in Mass and of Crystals and Crucible after dehydration
Crystal
Crucible
Mass change increase/decrease
0.9317 g decrease
0.320 g increase
Calculation
2.0002-1.0685=.9317
23.579-23.5758=0.00320
The masses in both lab show that there were chemical and physical reactions that took place. As seen in
table 1, the crystal’s weight comes from the potassium hydroxide and the sulfuric acid rather than the
aluminum. This was a chemical reaction. As seen in graph 1 and table 2, the crystals and crucible had a
mass change after being heated. This was a physical change. As seen in table 3, the crystal lost mass, and
Chemistry
October 16, 2013
the crucible gained mass. The crystals created were large and this was due to slow cooling of the
crystals. The dehydrated crystal became a bubble that filled with air and then hardened. The alum
crystals started out crushed with a mortar and pestle and became a liquid while heating and then when
all the water was gone, it hardened.
Discussion:
These results support my hypothesizes that were stated above. I feel that the results and hypothesizes
were similar because I had known what was supposed to happen in the lab. My hypothesis for the
dehydration of the crystals was also influenced by the title of dehydration. This allowed for me to easily
know that we were going to take the water out of the chemical formula.
Error Analysis:
Possible errors that could have occurred are over pouring of chemicals because I have not mastered that
skill yet, one of our beakers was dirty for the synthesis lab, the mass recorded for the crucible after
heating does not go out 4 places beyond the decimal like the other masses do, and systematically Kristin
and I forgot a step in the dehydration that caused us to start over and we did not have time to do the
second heating to make sure we had completely dehydrated the crystal. These errors could have
affected my results because there would be excess of chemicals and might produce more than
expected, the contaminated beaker may have allowed for the crystal to form on a particle left over and
that would cause it to grow as one big crystal like we had. Also, the systematic error of not taking the
proper mass would affect how accurate the results are when compared. We have nothing to compare
the mass to in order to know if we actually got all the water out or not. To reduce my error, I need to
make sure all beakers are clean before starting, pay closer attention to the balance when writing down
the masses, and work on pouring chemicals closer to the designated line it is supposed to be at. I also
need to make sure I read my lab manual in order.
Conclusion:
What has been learned in the synthesis lab is that when potassium and aluminum are mixed and you
add sulfate, a result of a crystal will occur. Also, I learned how to systematically use a funnel and funnel
paper. What was learned in the dehydration lab was that when you have a hydrate and it is heated the
hydrate evaporates leaving behind the pure chemical formula of the compound you just dehydrated.
The crystal became a looked and seemed like a completely different formula than when we started, but
it was only missing the water.