Synthesis and Analysis of Alum Ap Chem Lab

Synthesis and Analysis of Alum lab AP Chem.

       I.            Purpose: The purpose of this lab is to synthesize and later analyze alum. The purpose is also the calculate percent yield of the synthesized alum and to determine the chemical formula. The melting point and water of hydration of a sample of alum is determined. The percent sulfate is determined as well. These all help to figure out if the alum sample which was synthesized is really alum.

    II.            Materials:  Standard lab equipment was used to conduct the experiment. A Gooch crucible and Büchner funnel were used in the synthesis and in analyzing the alum. A capillary tube or a Thiele tube was used to determine melting point. Aluminum foil, 3 M sulfuric acid, 3 M potassium hydroxide solution, and aqueous ethanol solution were used to synthesize the alum.

 III.            Procedure: (15a): About 1.00 g of aluminum should be measured and torn into pieces. About 25 mL of KOH is added to the aluminum and then the solution is filtered in a Büchner funnel and filter flask. The solution should be allowed to cool. About 35 mL solution of 3M H2SO4 is added and stirred. If necessary, it should be filtered to remove any solid remaining. The solution should be boiled until 50 mL remain. It could be allowed to cool overnight or with an ice bath for 15 minutes. The alum crystals are collected by filtering with a Büchner funnel using 50 mL of ethanol-water solution. The crystals should be allowed to dry and the mass is then recorded.

(15b): To determine the melting point, a temperature probe should be used to measure when the alum solution melts. The alum should be pulverized and placed in a small capillary tube to a depth of about 1 cm. A rubber band should be used to hold the tube to the probe and then placed in a heated water bath and the melting point should be recorded. To determine the water of hydration, a crucible should be heated over a burner flame and the mass should be recorded after it is cooled. About 2 g of alum crystals are placed in a crucible and the mass is measured again. To dry, one can use a Bunsen burner and gently heat it until no vapor escapes from the crucible or place it in an oven overnight at 110°C. The crucible sample should then be massed, reheated for 5 minutes, and massed again. To determine the percent sulfate, a Gooch crucible should be obtained and massed. About 1 g of alum should be placed in a 250 mL beaker and 50mL of distilled water should be added. About 43 mL of Ba(NO­3)2 solution should be added to the solution. To prepare a precipitate one can use a ring stand and Bunsen burner and heat the beaker with a watch glass on top of the beaker for 15 near boiling. Or, a watch glass can be used to cover it and allowed to sit overnight. The solution should be then be filtered using a Gooch crucible, and the precipitate should be allowed to dry and massed.

 IV.            Data: Data is shown in the tables below and on the page from the lab notebook.


Method 1 – Overnight Cooling

Method 2 – Ice Water Bath

Mass of synthesized alum sample (g)



PART I – Melting Temperature Test Results

Method 1

Method 2

Control Sample

Melting Temperature (°C)




PART II – Water of Hydration Test Results

Method 1

Method 2

Control Sample

Mass of crucible and cover (g)




Mass of crucible, cover, and alum before heating (g)




Mass of crucible, cover, and alum after 1st heating (g)




Mass of crucible, cover, and alum after 2nd heating (g)




Mass of crucible, cover, and alum after final heating (g)




PART III – Percent Sulfate Test Results

Method 1

Method 2

Control Sample

Mass of Gooch filter and filter paper (g)




Mass of alum before reaction (g)




Mass of precipitate (g)




    V.            Calculations and Graphs: Some equations used were:

Percent Error = [(experimental value – accepted value) / (accepted value)] X 100%

Percent Yield = [(actual yield) / (theoretical yield)] X 100%

Ways these equations were used in the experiment:

To calculate the percent yield of the synthesized alum.

To calculate the percent error in melting temperature, water of hydration, and percent sulfate test results, in order to analyze the alum sample.

 VI.            Conclusion: The substance that we synthesized was alum, based on the percent error in our analysis of it. Since the percent error was relatively low, this means the measurements were fairly precise, and we can safely assume that the sample is alum. Both methods (overnight cooling and ice-water bath) of synthesizing alum gave us nearly the same results.

VII.            Discussion of Theory: This experiment required knowledge of being able to write and balance equations. It required knowing how to determine percent yield. The calculations show that we synthesized a relatively large amount of alum (compared to the theoretical yield), and overall it was pure. The purpose was fulfilled by determining whether the crystals created were actually alum. The experiment determined that yes, they were alum. 

VIII.            Experimental Sources of Error: Overall our results were fairly precise compared to the theoretical yield and the control sample of alum. However, some sources of error may have been: impurity of aluminum foil sample, improper techniques which resulted in loss of mass of alum sample when filtering, error in reading of measurements, etc. Also, human error is a given in any experiment, and is always accounted for.

 IX.            Questions


1.      Determine the theoretical yield of the alum. Use the aluminum foil as the limiting reagent and presume that the foil was pure aluminum. 14.015 g alum

2.      Calculate the percent yield of your alum crystals. Method 1: 84.00%. Method 2: 77.77%.

3.      Discuss the factors that affected the percent yield. Some factors that may have affected the percent yield are: loss of crystals during filtration, human error, error in measurement, and impurity of the aluminum foil.

4.      Write the balanced net ionic equations for the following: (a) aluminum and potassium hydroxide, yielding [Al(OH)4]- and hydrogen gas; (b) hydrogen ions and [Al(OH)4]-, yielding aluminum hydroxide; (c) aluminum hydroxide and hydrogen ions, yielding [Al(H2O)6]3+; and (d) the formation of alum from potassium ions, sulfate ions, [Al(H2O)6]3+, and water.

(a)   2Al(s) + 6H2O(l) + 2OH-(aq) à 2Al(OH)4-(aq) + 3H2(g)

(b)   H+(aq) + Al(OH)4-(aq) à H2O(l) + Al(OH)3(s)

(c)    Al(OH)3(s) + 3H2O(l) + 3H+(aq) à Al(H2O)63+(aq)

(d)   Al(H2O)63+(aq) + K+(aq) + 2SO42-(aq) + 6H2O(l) à KAl(SO4)2 • 12H2O(s)

5.      Describe a synthesis reaction. A synthesis reaction is one where a new substance is formed from the reaction of two or more other substances.

6.      Describe how the solubility of alum in various solvents and water at different temperatures was used in conducting the experiment. Since alum is soluble in water, we could not use water to filter it. So, an ethanol solution was used. Also, aluminum reacts with bases to produce aluminum hydroxide, which reacts with acids to produce [Al(H2O)6]3+, which is used to synthesize the alum.

7.      Write a detailed description of the alum crystals. They are white and semi-translucent, of various sizes, with flat sides.


1.      Is your sample alum? Use the results of the three tests to support your answer. Discuss the accuracy of your tests and possible sources of experimental error. Yes, our sample is alum. Although there is a relatively large percent error in the melting point tests, the other tests have much lower percent errors, which proves that our sample is indeed alum. Possible sources of error are improper techniques or human error, misreading measurements, and it is possible that the sample is alum but with other chemicals in it too. In other words, an impure sample, but still alum.

2.      Suggest other tests that could be conducted to verify the composition of your alum. Another test that could be used to determine if the sample is alum is to test its density. Density is unique for each substance; therefore it is an appropriate test.

3.      If the melting temperature test was the only test that you conducted, how confident would you be in the identification of your sample? Explain. We would be much less certain, because the melting temperature test was the one where we had the most error – where our data was the most imprecise in comparison to the control sample.

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