Integrated Science – Dr. May
Laboratory for the Synthesis of Alum
Objective:



Prepare Potassium Aluminum Sulfate (Alum) – KAl(SO4)2  12H2O
Study the stoichiometry of the synthesis
Calculate % yield
Procedure:
1. Label a 250 ml beaker with your group name and weigh it. Record the mass.
2. Obtain a sample of aluminum, cut the foil into small pieces, and record the mass
of the beaker plus aluminum.
3. Place about 50 ml of 1.5 M potassium hydroxide solution into the 250 ml beaker
with the aluminum.
4. As soon as gas evolution begins, place the reaction beaker in the fume hood for
about 15 minutes. The reaction which takes place is:
2Al + 2KOH + 6H2O  2KAl(OH)4 + 3H2 
5. Prepare a funnel and a folded filter paper and place it into a small ring on a ring
stand.
6. Under the funnel place a 250 ml beaker containing 25 ml of 8.6 M sulfuric acid
solution.
7. Filter the mixture into the sulfuric acid solution and slowly stir the white
precipitate. The two-step neutralization reaction is:
2 KAl(OH)4
2Al(OH)3
+
+
H2SO4 
3 H2SO4 
K2SO4
Al2(SO4)3
+
2 H2O
+
+
2Al(OH)3
6 H2 O
8. Label the beaker with your group name.
9. Place the beaker with the white precipitate on a hot plate and heat until the solid
completely dissolves.
10. Place the labeled beaker aside to slowly cool overnight. The solid formed is:
K2SO4
+
Al2(SO4)3 +
12 H2O 
2 KAl(SO4)2  12H2O
11. At the next laboratory period, decant the liquid from the crystals into a container
provided.
12. Obtain the mass of a small weighing dish.
13. Rinse the crystals with a small amount of cold water, collect the crystals, and
place them in the weighing dish to dry.
14. Obtain the mass of the crystals and record the data.
Data:
Mass of reaction beaker
Mass of reaction beaker plus aluminum
Mass of aluminum
Moles of aluminum
____________________________
____________________________
____________________________
____________________________
For each mole (or portion of a mole) of aluminum used, a mole (or equal molar amount)
of alum is formed.
Moles of alum (theoretical)
Mass of alum (theoretical)
Mass of plastic weighing dish
Mass of plastic weighing dish plus alum (wet)
Mass of plastic weighing dish plus alum (dry)
____________________________
____________________________
____________________________
____________________________
____________________________
Mass of alum (actual)
____________________________
Calculate the percent yield
____________________________
% yield =
Actual mass
Theoretical mass
x
100
Explain why the % yield is not 100%:
Complete and net reaction:
2Al + 2KOH + 6H2O  2KAl(OH)4 + 3H2 
2 KAl(OH)4 + H2SO4  K2SO4 + 2 H2O + 2Al(OH)3
2Al(OH)3 + 3 H2SO4  Al2(SO4)3 + 6 H2O
K2SO4 + Al2(SO4)3 + 12 H2O  2 KAl(SO4)2  12H2O