LAB – SYNTHESIS OF ALUM
Purpose: The primary purpose of this lab is to synthesize crystals of potassium aluminum sulfate
.
dodecahydrate (alum) [formula: KAl(SO4)2 12 H2O] from aluminum foil and a few other simple reagents.
The crystals produced will be weighed and stored for use in later investigations. This particular process
produces a surprisingly high yield of crystals for the simplicity of the reaction.
Materials: aluminum foil, 3 M H2SO4, 3 M KOH, 50%/50% water/ethanol solution
Equipment: 250 mL beaker, watch glass, 50 mL graduated cylinder, stirring rod, buchner funnel, 500 mL
filter flask w/ rubber tubing, ring stand, wire gauze, Bunsen burner, iron ring, plastic wrap, balance
Safety: both the sulfuric acid and the KOH can be damaging to skin and eyes. Neutralize any spills with
baking soda or vinegar
Procedure:
Day 1
1) Weigh out approximately 1 gram of aluminum foil. Record its exact mass in the data section. Tear the
foil into small pieces and place into the 250 mL beaker.
2) Slowly add 25 mL of 3 M KOH solution. Gently swirl the beaker while adding the KOH. This reaction
will produce some heat and hydrogen gas. Do not directly smell the vapors. The foil should completely
dissolve, leaving only a few small particles of carbon floating in the solution. If the amount of carbon
seems excessive, filter the solution with fine filter paper and the buchner funnel.
3) When the solution has cooled to room temperature, slowly add 35 mL of 3M H2SO4 with constant
stirring. The solution will get hot as the reaction proceeds. You should see solid Al(OH)3 precipitate
forming, which will then dissolve as more acid is added. If there is still precipitate present after all 35 mL
of acid has been added, keep adding acid until all the precipitate is gone.
4) Place the beaker on a ring stand, and boil the solution down until its volume is approximately 50 mL.
5) Gravity filter the hot solution to remove the carbon residue.
6) Cover the beaker with plastic wrap and set it aside to cool for the evening. Make sure to label the
beaker with your name. Your instructor may direct you to put your beaker in the refrigerator.
7) If no crystals have formed the next day, try scraping the bottom of the beaker with a glass stirring rod.
If there are still no crystals, try boiling off some more water and cooling again.
Day 2
8) Set up your vacuum filter by placing the buchner funnel in the mouth of the 500 mL filter flask. Make
sure there is a good seal. Attach the rubber tubing to the sidearm of the filter flask and then to the aspirator
on the sink at your lab table. Place a piece of round filter paper in the funnel. Turn on the sink to produce
the vacuum in the funnel. Check the quality of the vacuum by placing your entire hand over the opening of
the buchner funnel. If no vacuum is felt, check all the seals for leaks.
9) Pour all of your crystals and remaining liquid from the beaker into the buchner funnel. Once all the
liquid is all drawn through, wash the crystals with 50 mL of the 50/50 water/ethanol mix.
10) Allow the crystals to dry at room temperature. When they are dry, measure the mass of the crystals
you produced.
11) Thoroughly clean your glassware and return your equipment to its proper place
Data:
Mass of aluminum foil
Mass of alum crystals
produced
LAB – SYNTHESIS OF ALUM
Calculations:
1) Write balanced equations for the following reactions which occurred during this synthesis:
a) Aluminum reacts with potassium hydroxide and water forming potassium ions,
tetrahydroxyaluminate ions [ Al(OH)4-1 ] and hydrogen gas.
b) Hydrogen ions from the acid react with the tetrahydroxyaluminate ions to form aluminum
hydroxide and water.
c) Aluminum hydroxide reacts with additional hydrogen ions and water to form a hydrated
aluminum ion [ Al(H2O)6+3 ].
d) Potassium aluminum sulfate (alum) forms from potassium ions, hydrated aluminum ions,
sulfate ions, and water.
2) Assuming aluminum foil was the limiting reactant, using the reactions above, determine the theoretical
yield of alum crystals in grams.
3) Using the actual mass of crystals you obtained, calculate the percent yield of alum for this lab.
Conclusions:
1) List any sources of error in this experiment. Why might your percent yield be less than 100%? How
could this have been improved?
2) What shape are your crystals?
3) Why would water be a poor choice to wash the alum crystals?