Synthesis and Analysis of Alum
Purpose


To synthesize and isolate a compound
To characterize the compound by determining melting point and waters of
hydration.
Introduction
In this experiment the ionic compound, commonly called alum, will be
precipitated from an aqueous solution. Alum is a double salt, containing both aluminum
and potassium ions along with sulfate ions and water of hydration. Its formula is
KAl(SO4)212H2O and its name is potassium aluminum sulfate dodecahydrate. Alum is
commonly used as a binding agent for dyes.
Aluminum is considered a reactive metal, but because its surface is usually
protected by a thin film of aluminum oxide, it reacts only slowly with acids. It does,
however, dissolve quickly in basic solutions. Excess hydroxide ion converts the
aluminum to the tetrahydroxoaluminate(III) ion, [Al(OH)4]-. When acid is slowly added
to this ion, white, gelatinous aluminum hydroxide (Al(OH)3) precipitates. Continued
addition of acid causes the precipitate to dissolve because hydroxide ions are completely
neutralized and the aluminum then exists in solution as the hydrated ion [Al(H2O)6]3+.
Aluminum hydroxide is considered to be an amphoteric hydroxide because it dissolves in
both acids and bases.
After a compound is synthesized, tests can be carried out to verify that the desired
compound has been produced or to determine the characteristics of a new compound. In
this experiment, the melting point and the amount of water of hydration present will be
determined.
Procedure
Part I Synthesis of Alum
1. Weigh out about 1 gram of aluminum foil to the nearest 0.001 g. Tear it into small
pieces and place it in a 250 mL beaker.
2. Slowly add 25 mL of 3 M KOH solution. Allow the reaction to proceed until all of
the foil is dissolved. Remove any undissolved solids such as carbon particles by
filtering the solution through a Buchner funnel while the solution is hot. Rinse the
filter paper with a small amount of distilled water. Pour clear filtrate into clean 250
mL beaker.
3. Allow the solution to cool and then acidify it slowly, with constant stirring, using 35
mL of 3 M H2SO4. The solution will get very hot because you are adding strong
acid to a strongly basic solution. You should see Al(OH)3 precipitate and then
dissolve again. If a precipitate still remains when all of the acid has been used, filter
the solution and discard the solids. (Use vacuum filtration to speed up the process.)
4. Boil the solution until the volume has been reduced to about 50 mL.
5. To crystallize the alum, cool the solution in an ice bath for 15 minutes, keeping it as
motionless as possible. If no crystals form, scrape the bottom and sides of the
beaker with a stirring rod to provide a rough place where crystals may begin to
Synthesis and Analysis of Alum
grow. If there are no crystals, reheat the solution until more water has evaporated
and then cool again.
6. Collect the alum crystals by vacuum filtration. Determine the mass of the filter
paper before placing it in the funnel. Wash the crystals with 50 mL of a 50% by
volume water and ethanol mixture, in which alum crystals are not very soluble. Pull
air through the crystals to help dry them. Transfer crystals and filter paper to a premassed watch glass. Allow the crystals to dry overnight at room temperature.
7. Determine the mass of the alum, filter paper and watch glass.
Part II Determination of the Melting Point of Alum
1. Pulverize a small amount (pea size) of dry alum. Use a mortar and pestle, or use a
watch glass and the bottom of a test tube. Pack the alum in a capillary tube to a
depth of about 1 cm. To get the alum into the capillary tube, push the open end of
the capillary down into a small pile of alum powder. Turn the tube so the open end
is up and bounce the bottom of the tube on the desk top.
2. Using a 1 mm length of rubber tubing as a rubber band, fasten the capillary tube to a
thermometer. The alum should be level with the bulb of the thermometer. Insert
the thermometer through a split cork and place in a Thiele melting point tube. Heat
the tube as indicated in the diagram. Record the temperature at which the alum
melts (the white powder becomes clear). Repeat the melting point determination
with a new sample and a new capillary tube.
Part III Determination of the Amount of Water of Hydration in Alum Crystals
1. Heat a crucible with a cover over a burner flame until it is red hot. Allow it to cool
and find the mass. Handle with tongs to avoid getting fingerprints on them.
2. Add about 2 g of alum crystals to the crucible and accurately determine the mass.
3. Set the crucible at an angle in a pipestem triangle, cover loosely with the crucible
cover and heat very gently. The alum crystals will melt and the water of hydration
will vaporize. After the vapor is apparently driven off, heat more strongly for five
minutes. Cool and find the mass of the crucible, cover and anhydrous alum.
Calculations
Part I
1. Assuming Al is the limiting reagent, calculate the theoretical yield of alum.
2. Calculate the percent yield.
Part II
1. Look up the accepted value for the melting point of alum and calculate the percent
error in your determination.
Part III
1. Calculate the moles of anhydrous alum remaining.
2. Calculate the moles of water that were driven off.
3. Calculate the mole ratio of water to anhydrous alum and write the formula for alum,
indicating the number of waters of hydration present.
Synthesis and Analysis of Alum
Data
Part I
Mass Al foil
Mass filter paper
Mass watch glass
Mass filter paper, watch glass & alum
Mass alum
Part II
Trial 1
Melting point of alum
Part III
Mass crucible & cover
Mass crucible, cover & alum
Mass crucible, cover & anhydrous alum
Mass anhydrous alum
Mass water lost
Results
Part I
1 Theoretical yield of alum
2 Percent yield
Part II
1 Percent error
Part III
1 Moles of anhydrous alum
2 Moles of water
3 Formula for alum
Trial 2
Synthesis and Analysis of Alum
Questions
1. List three reasons why you should not expect 100% yield? Be specific and explain.
2. Comment on the results of the different tests used to verify that the substance tested
was alum. How confident are you that you made alum?
3. Give three other tests that could be used to verify the composition and/or identity of
alum.