Percent Yield of Potassium Aluminum Sulfate
Introduction: In this experiment, the ionic compound potassium aluminum sulfate
dodecahydrate, commonly known as alum, will be prepared from a water solution that contains
potassium ions (K+), aluminum ions (Al3+) and sulfate ions (SO42-).
Alum has a wide variety of uses. One example of this is a styptic pencil, which contains
potassium aluminum sulfate. These pencils were applied to facial cuts after shaving to coagulate
blood. It is also used as the acidic component in certain baking powders.
In order to get aluminum ions in solution, small pieces of aluminum foil will be placed in a
solution of potassium hydroxide. Although aluminum hydroxide is considered insoluble, excess
hydroxide will cause the formation of tetrahydroxoaluminate (III) ion, [Al(OH)4]-, forming a
colorless solution.
The solution will then be neutralized by using sulfuric acid (H2SO4). This will add sulfate ions to
the solution in order to create alum crystals. As the solution is neutralized, the aluminum
hydroxide will begin to precipitate, but continuing to add acid will cause all of the hydroxide
ions to be completely neutralized. The Al+3 will exist as a hydrated ion, [Al(H2O)6]3+.
OVERALL NET REACTION:
2 Al(s) + 2KOH(aq) + 4 H2SO4 (aq) + 22 H2O(l)  2KAl(SO4) 2•12 H2O(s) + 3 H2(g)
Equipment:
250 mL glass beaker
50 mL graduated cylinder
Buchner funnel and filter flask OR
funnel and filter paper Ice bath, if needed
analytical balance mortar and pestle
stirring rod
beaker tongs 125 mL beaker
Whatman #1 Medium/Fast filter
paper
Burner, ring stand, gauze
watch glass
hot plate
fume hood
Chemicals:
Aluminum foil
3M H2SO4 sulfuric acid
baking soda
3M KOH, potassium hydroxide
50% by volume water-ethanol solution
Vinegar, dilute HC2H3O2
SAFETY
You will be using solutions with high concentrations of sulfuric acid and potassium
hydroxide, both of which are highly damaging to skin and eyes. Be careful when handling them.
If you spill any on yourself, wash off with lots of cold water. Neutralize sulfuric acid spills on
the counter with baking soda; neutralize any potassium hydroxide spills with vinegar.
When the aluminum dissolves in potassium hydroxide solution, hydrogen gas is
produced. make sure no flames are present during this stage of the procedure. This step should
be performed in a fume hood.
Wear CHEMICAL SPLASH GOGGLES with indirect vents and a Chemical-Resistant Apron at
all times.
Read over the lab before beginning. Create a data table before beginning.!!!!
Procedure:
STEP 1: Creation of Al+3 solution
a. Measure about 1 gram of aluminum foil and record the actual mass used in your data table.
Tear the foil into small pieces and place in a 250 mL beaker.
b. Prepare 25 mL of 3M KOH. Check your prelab for quantities. Slowly add 25 mL of 3 M KOH
solution in a fume hood. The reaction will be violent and about 3 minutes in length. Keep all
flames far away. Allow the reaction to proceed until all of the foil is dissolved. Record
observations in your data table.
c. Filter the solution to remove any solid particles. Rinse the filter paper with a small amount of
distilled water.
d. Allow the solution to cool for 5 minutes.
STEP 2: Acidification through Addition of SO4-2 ions
a. While stirring, acidify the solution by slowly adding 35 mL of 3 M H2SO4. Neutralization is
very exothermic, so be prepared for this release of heat.
b. Solid Al(OH)3 will first start to precipitate and then dissolve as more acid is added. Heat
gently to dissolve completely . If all does not go into solution, then filter again. (Collect the
filtrate into a 125 mL beaker for the next step.)
c. Boil the solution until water has evaporated to a volume of about 50 mL of solution.
*If time permits, continue with step 3 in the same class period. Otherwise, cover the
beaker with a watch glass and allow to sit overnight.
STEP 3: Alum crystallization
a. Cool the solution in an ice bath for 15 minutes, keeping it as motionless as possible. Crystals
of alum should begin to grow in the beaker. If not, scrape the bottom of the beaker with a
stirring rod.
b. Filter the alum crystals through Whatman #1 filter paper and then wash with 50 mL of a 50%
by volume water-ethanol mixture.
c. Allow the crystals to dry overnight.
d. Determine the mass of the alum and record the mass in your data table.
Calculations:
1. Calculate the theoretical yield of potassium aluminum sulfate using the mass of aluminum foil
as the limiting reagent.
2. Calculate the percent yield.
Data Table:
Qualitative Data
Quantitative Data
Discussion: Answer the following questions:
1. What was the percent yield?
* Show all stoichiometry, units, and significant figures. Box your final answer.
2. Discuss what effect errors had on data.
3. One qualitative test for ionic solids is that they dissolve in water but not in alcohol, while
molecular substances could dissolve in both. In step 3b, you used a 50-50% water-alcohol
solution to wash the crystals. Why did the crystals require washing?
Conclusion:
Write a one paragraph summary of the entire experiment. Include the purpose of the experiment,
your percent yield, and an explanation of the loss in product.
Objectives of Lab
* % yield (weigh out aluminum accurately to then calculate theoretical yield using
stoichiometry)
*safety procedure: use of fume hood, height of sash
*technique: filtration, hygroscopic substances
*disposal procedures: of acid (neutralize it with base, test with baking soda)
*error analysis development
Teacher Notes
An alternate experiment would use an aluminum soda can.
Disposal of sulfuric acid, Flinn 24.b, not down sink.
Addition of KOH must be done under fume hood, violent reaction for 3 minutes.
Mixing of solutions: suggestions
* students prepare 3M KOH, teacher may prepare to save time
Conclusion:
Write a one paragraph summary of the entire experiment. Include the purpose of the experiment,
any major findings, and an explanation of the findings.