Experiment
4
Synthesis and Analysis of Alum,
AlK(SO4)2∙12H2O
Introduction:
When a compound is synthesized in a laboratory setting, tests are performed to ensure the
compound made is the desired product. This lab will familiarize you with a simple
procedure for synthesizing one common compound as well as several confirmatory tests
involving the compound aluminum potassium sulfate (a.k.a. alum).
Background:
Each compound has a unique set of properties that may be used to positively identify the
substance. One test alone is not enough to confirm or disprove the synthesis of a
substance, so the tests are performed in combination with one another to experimentally
prove the identity of any given substance. We will be testing for the melting point, the
number of moles of water in the hydrate, and the percent composition of sulfate within
the compound.
Overview:
Our purpose is to first synthesize alum using a series of simple chemical reactions. After
the alum has been synthesized, we will analyze the alum by three techniques for the
purpose of confirming its identity. The three properties are melting point, the number of
moles of water in the hydrate, and the percent composition of sulfate within the
compound. Upon completion of the lab, you will compare your results to the literature
value of each property of alum.
Pre-Lab Questions:
1. When measuring a melting point, why is it necessary to raise the temperature
very slowly when approaching the melting point temperature?
2. Washing soda is a hydrated compound with a formula Na2CO3∙xH2O, where x
is the number of moles of water per mole of Na2CO3. When a 2.123g sample
of washing soda was heated at a temperature of 130oC, all the water of
hydration was driven off, leaving 0.787g of anhydrous sodium carbonate.
Calculate the value of x.
3. The formula of Epsom salt is MgSO4∙7H2O. If 1.250g of the compound is
dissolved in water, calculate the number of milliliters of 0.200 M Ba(NO3)2
that would be required to precipitate all of the sodium ions as barium sulfate.
4. Make the same determination as in question 3 for 1.000g of alum.
Gates Chili AP Chemistry
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Experiment 4
Materials: aluminum foil, 3 M sulfuric acid, sodium hydrogen carbonate, 3 M potassium
hydroxide solution, aqueous ethanol (50%), dilute acetic acid solution, ice bath, Bunsen
burner, capillary tube, mortar and pestle, ring stand, iron ring, wire gauze, thermometer,
rubber band, clamp, beaker, split stopper, Mettler balance, crucible and cover, pipestem
triangle, aspirator, filter flask or filter funnel, quantitative filter paper, graduated cylinder,
stirring rod, wash bottle, watch glass
Safety: Barium nitrate is toxic by ingestion, sulfuric acid, potassium hydroxide, and
dilute acetic acid are caustic and can cause burns.
Procedure:
Synthesis of Alum:
1. Obtain a piece of aluminum foil and determine its mass on an analytical balance.
Try to use approximately 1 gram of aluminum.
2. Tear the foil into small pieces and place into a 250 mL beaker.
3. Prepare a Buchner funnel and flask apparatus to be used in step 5.
4. Measure 25 mL of 3 M KOH solution. Slowly add the KOH solution to the beaker
containing the aluminum pieces. Allow the reaction to proceed until all aluminum
is dissolved.
5. Carefully pour the reaction mixture through the Buchner funnel and rinse the filter
paper with a small amount of distilled water.
6. Rinse the beaker with distilled water and pour the filtered liquid back into the
beaker.
7. Allow the solution to cool to room temperature (this may be left overnight as long
as the container is sealed with parafilm).
8. Clean the Buchner funnel and flask and prepare for use again.
9. Measure 35 mL of 3 M H2SO4 solution. Be sure the reaction mixture has cooled
to room temperature before proceeding.
10. Slowly add the sulfuric acid solution to the reaction mixture. Stir continuously. If
any solid remains after the entire 35 mL of sulfuric acid solution has been added,
pour the mixture through the Buchner funnel and flask apparatus to separate the
solid particles from the mixture.
11. Gently boil the reaction mixture to a volume of approximately 50 mL.
12. Cool the reaction mixture by either allowing it to sit overnight (recommended for
best crystallization of alum) or by placing the reaction beaker in an ice bath. The
reaction beaker should remain uncovered. Crystals will begin to form within 15
minutes. If crystals do not form within this time, add a single seed crystal of alum
to the beaker or heat the solution to remove more water.
13. Collect the alum crystals by pouring them into a Buchner funnel and flask
apparatus. Use vacuum filtration and wash the crystals with 50 mL of a 50%
aqueous ethanol solution.
14. Remove the filter paper and crystals from the Buchner funnel and allow the
crystals to dry at room temperature. Measure and record the mass of the alum.
Store the crystals in a separate, labeled container for analysis in the next part of
this lab.
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Gates Chili AP Chemistry
The Synthesis and Analysis of Alum
Analysis of Alum:
Part I:
1. Use a mortar and pestle to pulverize a small amount (about 0.5g) of dry alum.
2. Pack the alum in a capillary tube to a depth of approximately 0.5 cm.
3. Pack the alum tightly against one end of tapping the end of the tube gently
against the table.
4. Fasten the capillary tube to a thermometer with a rubber band. The alum in the
capillary tube should be level with the bulb of the thermometer.
5. Use a clamp and a split rubber stopper to fasten the thermometer to a ring
stand.
6. Immerse the bottom of the thermometer and capillary tube into a beaker of
water and heat. Stir the water to maintain an even temperature. Slow the
heating as you near the melting point of alum to ensure an accurate reading of
melting point temperature.
7. Record the temperature range at which alum melts in your data table.
8. Repeat this procedure using a fresh sample of alum and a new capillary tube.
Part II:
1. Set up a ring stand with an iron ring and pipestem triangle. Place your lab
burner beneath the apparatus.
2. Adjust the height of the iron ring so the crucible you will place on the
pipestem triangle will receive maximum heat from the burner.
3. Wash and dry the crucible by heating until it is red hot. Carefully remove the
crucible using tongs and place on the lab bench to cool. Complete cooling will
take approximately ten minutes.
4. Using the Mettler balance, determine and record the mass of the crucible and
cover.
5. Add approximately 2g of alum to the crucible. Cover the crucible and record
the mass determined by the Mettler balance.
6. Heat the crucible and alum crystals gently. As the crucible warms, tilt the
cover to allow maximum air flow to the alum crystals. The crystals will melt
under gentle heat and the water of hydration will be driven off.
7. When the bubbling stops, increase the heat slightly and allow the crystals to
be heated more strongly for about five minutes to ensure all the water of
hydration has been driven away.
8. Turn off the lab burner and allow the crucible to cool. You may wish to use a
desiccator to prevent moisture from the air to absorb into the anhydrous alum
crystals.
9. After cooling is complete (about 10 minutes) determine and record the mass
of the crucible and anhydrous alum crystals.
10. Dispose of the alum crystals as instructed and clean and replace the crucible
and cover.
Gates Chili AP Chemistry
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Experiment 4
Part III:
1. Connect the Gooch crucible or filter funnel to the filtering flask and aspirator
on the lab faucet.
2. Allow the vacuum to pull about 50mL of water through the filter. Turn off the
vacuum, remove the crucible and dry the crucible for about 15 minutes at
120oC. Allow the crucible to cool, then determine and record its mass using
the Mettler balance.
3. Use the Mettler balance to accurately mass about 1g of alum into a clean 250mL beaker.
4. Dissolve the alum into about 50mL of distilled water.
5. Obtain 50 mL of a 0.200 M Ba(NO3)2 solution in a clean 50 mL graduated
cylinder.
6. Add twice the volume of Ba(NO3)2 as was calculated in Pre-Lab question #3
slowly while continually stirring the alum solution.
7. Cover the beaker with a watch glass and heat the solution to near its boiling
point. Keep the solution at just below its boiling point for 15 minutes. As this
occurs, precipitate particles will grow to a size that will allow them to be
filtered. If time is short, cover the beaker and allow the precipitate to stand
overnight.
8. Filter the precipitate through the Gooch crucible with suction from the
aspirator. Be careful not to overfill the crucible, barium nitrate has a tendency
to “creep” up the sides of its container.
9. Wash the beaker and precipitate several times with small quantities of distilled
water.
10. Allow the Gooch crucible to dry for at least 15 minutes at 120oC. Once the
crucible has cooled, determine and record its mass.
Disposal:
1. Place the capillary tubes and anhydrous alum in the chemical disposal
container for Parts I and II.
2. Place the precipitated barium sulfate and the filtrate in the disposal container
for Part III. The filtrate will contain excess barium nitrate as well as potassium
and aluminum ions.
Data:
Synthesis of Alum
Mass of aluminum foil
Mass of alum crystals
_______
_______
Description of alum crystals:
__________________________________________________________________
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Gates Chili AP Chemistry
The Synthesis and Analysis of Alum
Analysis of Alum
Part I:
Trial #1
Trial #2
Measured MP
Literature MP
Part II:
Mass of crucible and cover
Mass of crucible, cover, and alum
Mass of alum
Mass of crucible, cover and alum after heat
Mass of water driven off
Mass of anhydrous alum
Moles H2O
Moles AlK(SO4)2
Mole ratio; moles H2O:moles AlK(SO4)2
Gates Chili AP Chemistry
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Experiment 4
Part III:
Mass of alum
Mass of dry Gooch crucible
Mass of Gooch crucible plus barium sulfate
Mass of barium sulfate
Mass of sulfate in precipitate
Experimental percent of sulfate in alum
Theoretical percent of sulfate in alum
Calculations and Analysis:
1. Determine the theoretical yield for alum assuming aluminum is the limiting
reactant and that aluminum foil is pure aluminum.
2. Determine the percent yield of your alum crystals.
3. Find the literature value for the melting point of aluminum potassium sulfate
and record this value on your data table. Cite your source.
4. From the mass of anhydrous alum remaining in the crucible after heating and
the formula of alum, calculate the moles of anhydrous alum in the original
sample. Enter this value in your data table.
5. From the mass of water driven off from the sample and the molar mass of
water, calculate the moles of water in the original sample. Record this value in
your data table.
6. Calculate the mole ratio of water to anhydrous alum in the sample. Record
your results in the data table.
7. Calculate the percent sulfate in barium sulfate.
8. Calculate the mass of sulfate in the precipitated barium sulfate. Record this
value in your data table.
9. Calculate the percent sulfate in the alum sample. Record this value.
10. Calculate the theoretical percent sulfate in alum from its formula. Record this
value.
11. Calculate your percent error in the determination of sulfate ion in alum.
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Gates Chili AP Chemistry
The Synthesis and Analysis of Alum
Post-Lab questions:
1. Why must objects be cooled before their mass is determined on an analytical
balance?
2. Comment on the results of the different tests used to verify that the sample
tested was alum.
3. What other tests could be used to verify the composition of alum?
4. Write a concluding paragraph evaluating your execution of the procedure.
Include sources of error, the effects of the error on your results (particular
your percent yield results), and improvements that should be included to
increase the accuracy of your results.
Gates Chili AP Chemistry
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