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UNIT 1 Synthesis of Alum from Aluminum



1.0  Introduction

2.0  Objectives

  1. Main Content
  2. Principle      
  3. Experimental Procedure

4.0  Conclusion

5.0  Summary

6.0  Tutor Marked Assignments

7.0  References/Further Reading




Alums are ionic compounds that crystallize from solutions containing sulfate ion, a trivalent cation

3+  3+  3+  +  +  + such as Al , Cr , or Fe  and a monovalent cation such as K , Na , or NH . Six of the water

4 molecules bind tightly to the trivalent metal ion; the remaining six molecules bind more loosely to the monovalent cation and the sulfate anion.

In this experiment you will prepare an alum–KAl(SO ) ·12H O [potassium aluminum sulfate

 4 2  2

dodecahydrate]–from an aluminum beverage can. This compound is widely used in dyeing fabrics, making pickles, making paper and purifying water.

Aluminum is the most abundant metal in the earth’s crust (8.3% by weight) and is the third most abundant element after oxygen (45.5%) and silicon (25.7%). Pure aluminum is a silvery-white metal and has many desirable physical and chemical properties: it is light-weight, non-toxic, corrosion-resistant, nonmagnetic, and malleable. Aluminum is commonly combined with other metals such as copper, manganese, silicon, magnesium, and zinc, which produces alloys with high mechanical and tensile strength. You are probably familiar with many uses of aluminum and its alloys.




By the end of this unit, you should be able to:

  • Use aluminum from an aluminum can to synthesize a chemical compound, alum, (hydrated potassium aluminum sulfate,Image From From EcoleBooks.comO).
  • jPerform some stoichiometry calculations, specifically the percent yield of product.






  1. Theory: Aluminum beverage cans generally have a thin coating of plastic on the inside that protects the
    aluminum from the corrosive action of the chemicals in the beverage. The outside usually has a thin coating of paint. These coatings must be removed before any chemical reactions with the metal can be carried out. The coatings may be effectively scraped off with a metal pan cleaner. A cleaned piece of metal is then dissolved in a potassium hydroxide solution according to the following complete, balanced equation:


The full and net ionic equations are:


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 +  + Image From EcoleBooks.comImage From   Image From  +Image From  



The Image From ion is a complex ion called “aluminate.” After filtration to remove residual plastic and paint decomposition products, the alkaline solution of Image From – is clear and colorless. The Image From is evolved as a gas and mixes with the atmosphere. The chemical species in solution are

potassium ions (Image From and aluminate ions [Image From] -] ions (plus any unreacted KOH).

Sulfuric acid is now added and two sequential reactions occur. Initially, before the addition of all the acid, insoluble aluminum hydroxide is formed,


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+ +


Image From EcoleBooks.comto give a thick, white, gelatinous precipitate of aluminum hydroxide. As more sulfuric

acid is added, the precipitate of Image From dissolves to form soluble Image From EcoleBooks.comions.


The full and net ionic equations are:


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  +Image From  +  


to give aluminum ions, Image From, in solution. The solution at this point contains Image From EcoleBooks.comions, Image From ions (from potassium hydroxide), and Image From EcoleBooks.comions (from sulfuric acid). On cooling, crystals of hydrated potassium aluminum sulfate, Image From From EcoleBooks.comO (or alum) are very slowly deposited.

In the experiment the crystallization process is speeded up by providing a small “seed crystal” of alum for the newly forming crystals to grow on. Cooling is needed because alum crystals are soluble in water at room temperature.


The full and net ionic equations are:


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Finally, the crystals of alum are removed from the solution by vacuum filtration and washed with an alcohol/water mixture. This wash liquid removes any contamination from the crystals but does not dissolve them. It also helps to dry the crystals quickly, because alcohol is more volatile than water.




3.2 Experimental Procedure

CAUTION!! Excess care must be used in handling potassium hydroxide (KOH) and sulfuric acid (H2SO4). KOH is corrosive and will dissolve clothing and skin! Sulfuric acid will also burn and dissolve clothing. Wash your hands thoroughly after using either of these solutions!


A piece of scrap aluminum will be provided. Use steel wool to remove as much paint as possible. The inside of the can is protected with a plastic coating and you should remove this also. Weigh the cleaned strip of aluminum to the nearest 0.001 grams.


Cut the Al sample into small squares and place the squares in a clean 100-mL or 150-mL beaker. Perform the following in the hood!! Carefully add 20 mL of 4 M potassium hydroxide, KOH. Bubbles of hydrogen gas should evolve. Place your beaker on a hot plate in the hood to speed up the reaction. When hydrogen bubbles are no longer formed, the reaction is complete. This should take 10-15 minutes. Remove the beaker from the hot plate and allow it to cool at your bench.


The resulting grayish mixture should be filtered to remove unwanted impurities. If the solution is still warm, cool it by placing the beaker in an ice bath. Set up a 250-mL filter flask and Gooch filter crucible or Büchner funnel as demonstrated. Don’t forget to clamp the filter flask to some kind of support. Filter the solution slowly. Rinse the beaker two times with small portions (<5 mL) of distilled water, pouring each rinse through the filter. Transfer the clear colorless filtrate to a clean 250-mL beaker.


To the cool solution, slowly and carefully, with stirring, add 15 mL of 6 M sulfuric acid. White lumps of Al(OH)3
should form in the solution . Again working in the hood, heat and stir the mixture to dissolve the white lumps. Excess sulfuric acid may be added dropwise, but no more than 30 mL total, in order to totally dissolve the Al(OH)3, and give a clear solution. Cool the clear solution in an ice bath for at least 20 minutes. Crystals of alum should fall to the bottom of the beaker. If no crystals form, scratch the bottom of the beaker with your stirring rod. If that fails, add a “seed” crystal of alum to facilitate crystallization.


While the solution is cooling, reassemble the filtration apparatus. Be sure to weigh the filter paper at this point. Slowly filter the solution containing the crystals. Rinse the beaker once with 5 mL of cool distilled water and once with 5 mL of isopropyl (rubbing) alcohol, pouring each rinse through the filter. Allow the aspirator to pull air through the crystals until they appear dry, at least 5 minutes. Remove the filter from the flask. More crystals may form in the filtered solution (filtrate). If you have time, filter these crystals as just described, and add these to the first crystal crop.


Remove the damp crystals and filter paper from the filter and place them into a weighed beaker. Place the beaker containing your crystals in your locker until next week. After drying, weigh the filter and crystals,and determine the mass of the alum produced. Calculate the percent yield.


Determine the melting point of the alum crystals using the following procedure. Carefully, push the open end of a capillary tube into your crystals, forcing some of the solid into the tube. Turn the tube over and tap it gently to move the crystals to the sealed end of the tube. Repeat this process until you have about 5 mm of solid in the capillary tube. Carefully, insert the tube into the melting point apparatus. The apparatus will be HOT if others have been using it! Note the temperatures at which the alum first appears to melt (i.e. when liquid first becomes visible) and at which it is completely melted (all liquid). Discard the capillary tube in the glass bin.


Obtain the literature value for the melting point of alum. Compare your experimental melting point to the literature value. Calculate the percent error.





Prepare a brief report describing your observations during the experiment. Report the actual yield, the theoretical yield and the percent yield



Image From

Alum is a potassium aluminum sulfate dodecahydrate. It is a white crystal that can be used for water purification, leather tanning, as an astringent, and in baking powder. Alum is made from scrap aluminum metal.



The stoichiometry involved in the sequence of reactions leading to the preparation of alum provides the mole relationship between aluminum and alum that is required to calculate the percentage yield.


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Image From From Image From + Image From EcoleBooks.comImage From From EcoleBooks.comO + Image From (5)




Al, 26.98 g/mol KAl(SO4)2•12H2O, 474.39 g/mol



The overall reaction for the synthesis of alum, equation (5), is obtained by adding reactions (14) and canceling like-species. The overall reaction stoichiometry (5) informs us that 2 moles of aluminum will produce 2 moles of alum.


Your calculations should proceed as follows:

  1. Calculate the number of moles of Al used from the mass of Al used.
  2. Knowing that the stoichiometric factor is


    Stoichiometric factor
    Image From



    calculate the quantity of alum),Image From From EcoleBooks.comO in moles, that should be produced theoretically from the quantity (in moles) of aluminum metal used.

    Enter this calculation on your report form.

  3. Knowing the number of moles ofImage From
    Image From From EcoleBooks.comO expected, calculate the mass of Image From From EcoleBooks.comO expected (the theoretical yield). Enter this on your report form. d) Calculate the percentage yield of alum, if

    % yield = Image From


    Enter the result of your percent yield calculation on your report.


Give the completed report form to your laboratory instructor by the indicated due date. Be sure that your instructor also has the sample of the alum that you prepared.



Mass of aluminum used ________________________________

Mass of paper or beaker + Image From From EcoleBooks.comO ______________________

Mass of paper or beaker ______________________

Mass of Image From From EcoleBooks.comO ______________________




Show your calculations throughout!

  • How many moles of aluminum metal did you use?
  • Assuming that Al metal is the limiting reagent, what quantity of potassium aluminum sulfate (alum), in moles, is theoretically obtainable?
  • What mass of potassium aluminum sulfate (alum, Image From From EcoleBooks.comO), in grams, is theoretically obtainable from your mass of Al? That is, what is the theoretical yield of alum?
  • What was the percentage yield of your product, Image From From EcoleBooks.comO?

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EcoleBooks | UNIT 1           Synthesis of Alum from Aluminum - INORGANIC CHEMISTRY PRACTICALS


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