Analysis of Alum
KAl(SO4)2.12H2O
After a compound has been synthesized, tests should be carried out to verify
the compound formed is indeed the compound desired. There are a number of
various tests that can be performed to verify that the compound is the one desired.
In a previous experiment, alum crystals were prepared. In this experiment we will
do several tests to determine if the crystals are really alum.
The first and simplest test is to find the melting point of the compound and
compare it to the published value for alum. A small quantity of alum is powered and
placed on a melting point apparatus. The crystals are heated on the melting point
apparatus on a hot plate, and the temperature at which they melt is recorded and
compared to the reported values.
The second test that we can do is to determine the amount of water of
hydration present in the alum crystals. Some of the alum is placed in a crucible and
weighed. The crucible is heated until all of the water of hydration is driven off. The
crucible is then cooled and its mass measured. From the mass of the dry crystals
and the mass of the water lost, the ratio of moles water to moles KAl(SO4)2 can be
calculated and then compared to the correct formula values.
The third test is a chemical test to determine the percent of sulfate in the
compound prepared. A weighed quantity of alum is dissolved in distilled water. An
excess of barium ions is added to the solution to precipitate all of the sulfate in the
compound as barium sulfate. The precipitated barium sulfate is filtered off, dried,
and its mass determined. From the mass of the barium sulfate and the initial mass
of the alum, the percent sulfate can be calculated and compared with the theoretical
percent found from the formula.
Chemicals
Parts 1, 2, and 3
Alum crystals prepared by you in the Lab
Known Alum crystals
Part 3
Barium nitrate, Ba(NO3)2, 0.2M
Equipment
Part 1
Part 2
Capillary Tube
Melting point apparatus
Hot plate
Thermometer
Mortar and pestle
Crucible and cover
Clay triangle
Ring stand, ring and Bunsen burner
Part 3
Beaker, 250 mL
Stirring rod
Rubber policeman
Watch glass
Graduated cylinder, 100 mL
Ring stand, ring, Bunsen burner
Wash bottle with distilled water
Funnel, filter paper and flask
Procedure
Part 1: Find the Melting Point of Alum
Pulverize a small amount of dry alum. Use a mortar and pestle. Place a small
amount of alum on the top of the melting point apparatus and place the
thermometer in the hole of the apparatus. Place the melting point apparatus in a
hot plate and turn the heat to medium. Heat slowly to get an accurate value for the
melting point. When the crystals start to appear wet, you have reached the melting
point. Repeat the procedure with the commercial sample of alum. Find the
published value for the melting point of alum, and compare the experimental and
published values.
Part 2: Determine the Amount of Water of Hydration I Alum Crystals
Heat a crucible with a cover over a burner flame until it is red hot. Allow
them to cool, and find their mass using a sensitive balance. Handle with tongs to
avoid getting fingerprints on them. Now add about 2 g of alum crystals to the
crucible. Accurately determine the mass of the crucible, cover and crystals.
Set the crucible at an angle in a triangle held in a ring on a ring stand, cover
loosely with the crucible cover, and heat very gently. The alum crystals will melt,
and the water of hydration will vaporize. It is important that the escaping vapor
does not carry any of the anhydrous alum along with it, so be sure that the crystals
are heated very gently at first. 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.
Calculate the mass of the anhydrous alum and the mass of the water that was
driven off. Find the moles of anhydrous KAl(SO4)2, and the moles of water.
Calculate the ratio of moles water: moles KAl(SO4)2. And give the calculated formula
of alum, KAl(SO4)2.XH2O, where X=the ratio of moles of water: moles of alum.
Repeat the procedure with the commercial alum. Compare your value with that of
the published value for alum.
Part 3: Determine the Percent Sulfate in Alum
Use a sensitive balance to accurately weigh about 1 gram of alum in a 250 mL
beaker. Dissolve in approximately 50 mL of distilled water. Calculate the volume of
0.2 M Ba(NO3)2 which would be needed to totally precipitate all of the sulfate ion
present in solution, and then add twice this volume of Ba(NO3)2 slowly, while
stirring.
Cover the beaker with a watch glass and heat the solution nearly to boiling.
Keep the solution just boiling point for at least 15 minutes. During this time, the
precipitate particles grow to a filterable size. Alternatively, cover the beaker and
allow to stand overnight.
Filter the precipitate through Whatman No. 42 fine grained filter paper. Do
not over fill the filter paper. Use a rubber policeman to be sure that every particle is
transferred from the beaker and stiffing rod into the filter paper. Wash the beaker
and the precipitate several times with small quantities of distilled water.
Carefully transfer the filter paper and precipitate to a watch glass and dry it
in an oven set to 50oC. Allow the dry filter paper to cool and determine its mass.
Calculate the percent sulfate in the alum and compare to the value calculated from
its formula. Repeat the procedure with the commercial alum. Calculate to accepted
value for the percent sulfate in the compound and compare.
Questions:
report.
Answer each of the following in your lab
1. Why must objects be cooled before their mass is found on a sensitive
balance.
2. Comment on the results of the different tests yu used to verify that the
substance tested was alum.
3. What other tests could be made to verify the composition of alum?