Experiment 7: Synthesis of Alum
Purpose
The purpose of this experiment is to introduce the student to the concept of multi-step synthesis and lab procedures including
filtration, purification, crystallization, and melting point determination.
Background
In this experiment, scrap aluminum metal will be converted to the compound called alum. The name alum commonly refers
to potassium aluminum sulfate dodecahydrate, KAl(SO4)2·12H2O, which is a crystalline white solid. Among the many uses
for alum are water purification, leather tanning, mordant dyeing, and as a component in baking powder.
A synthesis may involve one chemical reaction or a series of chemical reactions. The series of reactions involved in the
synthesis of alum can be described by Eqs. (1), (2), (3), and (4).
2 Al(s) + 2 KOH(aq) + 6 H2O(l) ---> 2 K[Al(OH)4](aq) + 3 H2(g)
(1)
2 K[Al(OH)4](aq) + H2SO4(aq) ---> 2 Al(OH)3(s) + K2SO4(aq) + 2 H2O(l)
(2)
2 Al(OH)3(s) + 3 H2SO4(aq) ---> Al2(SO4)3(aq) + 6 H2O(l)
(3)
K2SO4(aq) + Al2(SO4)3(aq) + 24 H2O(l) ---> 2 KAl(SO4)2·12H2O(s)
(4)
A single overall equation {Eq. 5} for the chemical conversion of aluminum to alum can be written from a combination of
equations (1), (2), (3), and (4).
2 Al(s) + 2 KOH(aq) + 4 H2SO4(aq) + 22 H2O(l) ---> 2 KAl(SO4)2·12H2O(s) + 3 H2(g)
(5)
Sample Problem #1: Calculating Theoretical Yield
Calculate the theoretical yield of CaSiO3, if 62.04 grams of Ca3(PO4)2 is allowed to react with excess SiO2 according to the
following reaction.
2 Ca(PO4)2(s) + 6 SiO2(s) + 10 C(s)  6 CaSiO3(s)+ 10 CO + P4(s)
Step 1: Calculate the number of moles of Ca 3(PO4)2.
[62.04 g Ca3(PO4)2][1 mol Ca3(PO4)2 / 310.18 g Ca3(PO4)2] = 0.2000 mol Ca3(PO4)2
Step 2: Calculate the number of moles of CaSiO3, using the reaction stoichiometry.
[0.2000 mol Ca3(PO4)2][6 mol CaSiO3 / 2 mol Ca(PO4)2] = 0.6000 mol CaSiO3
Step 3: Calculate the mass of CaSiO3.
[0.6000 mol SiO2][116.17 g CaSiO3 / 1 mol CaSiO3] = 69.70 g CaSiO3
Sample Problem #2: Calculating Percent Yield
If only 57.2 g of CaSiO3 were collected from the reaction in Sample Problem #1, calculate the percent yield.
Percent yield = [Actual Yield / Theoretical Yield][100] = [57.2 g / 69.7 g][100] = 82.1%
25
Put on your CHEMICAL SPLASH SAFETY GOGGLES!
Experiment Procedures
Part 1. Synthesis, isolation, and purification of Alum
1.
Weigh out about 1g (±0.01 g) of aluminum metal and place the aluminum into a marked * 250 mL Erlenmeyer flask.
Record the mass of the aluminum. (*Use a pencil to label the Erlenmeyer flask in the white etched area on the flask.)
2.
CAREFULLY add 25 mL of 4 M potassium hydroxide, KOH, to the flask containing the aluminum.
CAUTION: KOH is a caustic material and is harmful to skin! Immediately
rinse affected area with plenty of water if the skin comes in contact with KOH.
3.
Place the marked flask under the fume hood and allow the reaction to proceed until all the aluminum has been digested.
This should take about 15 minutes. If after this period of time the aluminum is not all digested, warm the flask over a
small Bunsen burner flame. (Note: The aluminum used in this experiment is an alloy and contains other materials which
will not dissolve. After all the aluminum has been digested, the flask will contain a black, gritty material which will not
dissolve.)
CAUTION: The reaction is exothermic so the flask will get hot where the solution
is in contact with the flask. Also, hydrogen gas is evolved in this reaction.
4.
As demonstrated by the lab assistant, use a long stem funnel, and a piece of #4 filter paper, to gravity filter the warm
reaction solution into a 250 mL beaker. This will remove any insoluble impurities. Using iron tongs, remove the filter
paper and place in the trash can. Allow the clear solution to cool.
Use CAUTION when filtering the solution. Both the solution and flask may be hot!
5.
With stirring, slowly add 30 mL of 6 M H2SO4 to the cool solution. Solid aluminum hydroxide may begin to form.
6.
If needed, GENTLY heat the reaction mixture with stirring until all of the aluminum hydroxide dissolves.
7.
If there are no solid impurities present, proceed to the next step. If solid impurities are present, gravity filter the solution
again using a clean piece of filter paper.
8.
Cool the beaker and solution in an ice bath. Alum crystals should form within 20 minutes. If crystals do not form,
gently heat to reduce the volume by about one-half and then cool in the ice bath.
9.
Using a Buchner funnel, isolate the alum crystals and wash the crystals by pouring about 20 mL of 50% ethanol over
them in the filter while air is being drawn over the crystals. After washing, continue drawing air over the crystals for
about 3-5 minutes. Dispose of the liquid filtrate from the Buchner flask into the waste bucket under the hood.
10. Remove the crystals from the Buchner funnel and further dry them by pressing them between the folds of paper towel.
Dispose of the filter paper in the trash can.
11. Weigh and record the mass of alum isolated.
Part 2. Melting Point Determination
1. Place a small amount of the alum into the bottom of a melting point capillary tube.
2.
Secure the capillary tube to the bulb of a thermometer with a rubber band and set up a melting point apparatus as
demonstrated by the lab assistant.
3.
Slowly heat the water so that the temperature increases at a rate of about 3 o per min.
4.
Watch the solid in the capillary tube and record the temperature when the alum first begins to melt and the temperature
when the last crystal of alum melts. The literature value for the melting point of alum is 92.5 oC.
5.
Dispose of the capillary tube in the broken glass container.
6.
Place the remaining alum in the glass jar labeled “Student Prep Alum.”
26
Experiment 7: Synthesis of Alum
Name___________________________________________ Hood No._______ Date ___________
Data
mass of aluminum starting material
mass of alum isolated
melting point of alum
_______________g
_______________g
_______________oC
Show work for calculation of theoretical yield of alum based upon the following equation.
2 Al(s) + 2 KOH(aq) + 4 H2SO4(aq) + 22 H2O(l) ---> 2 KAl(SO4)2·12H2O(s) + 3 H2(g)
Theoretical Yield:________________
Show work for the calculation of percent yield.
Percent Yield:________________
27
28
29
30