The objective of the laboratory is to synthesize alum (KAl(SO4)2.xH2O) from aluminum powder and to determine the proportion of water in the alum crystals. Alum is a product from the reaction between potassium hydroxide and sulfuric acid. The reaction include several steps, as followed:
Aluminum powder reacts with potassium hydroxide to generate Al(OH)4- ions and release hydrogen. 2 Al(s) + 2 KOH(aq) + 6 H2O 2 K[Al(OH)4](aq) + 3 H2 (g)
A gelatinous precipitate of aluminum hydroxide was created when sulfuric acid was added to the aqueous solution of Al(OH)4- ions. 2 K[Al(OH)4](aq) + H2SO4 (aq) 2 Al(OH)3 (s) + K2SO4 (aq) + 2 H2O In this case, x= nH2Ondry product= 0.2279180.2721258= 12.00. Concerning the second crucible, an amount of 0.5 g of alum was added to the crucible. After heating, there was 0.2496 g of contents (KAl(SO4)2) left in the crucible. That means there was 0.2504 g of H2O fully evaporating. In this case, x= nH2Ondry product= 0.2304180.2696258= 12.24. The average result of x: x= 12.00+ 12.242= 12.12. With calculations concerning the masses of contents in the crucibles before and after heating, it is observed that 12.12 molecules of water in a mole of alum. The general formula of alum, therefore, is KAl(SO4)2.12.12H2O. The literature value of portions of water molecules in alum is 12, which makes the formula of alum KAl(SO4)2.12H2O. The proximity of the calculated result and the literature result reflected to efficiency and accuracy of the laboratory. Through a series of chemical reactions, alum (the double salt with incorporated water molecules, with the calculated formula of KAl(SO4)2.12H2O) was formed from aluminum powder, potassium hydroxide and sulfuric acid. The reactions lead to the formation of alum are summarised as followed: (I) 2 Al(s) + 2 KOH(aq) + 6 H2O 2 K[Al(OH)4](aq) + 3 H2 (g) (II) 2 K[Al(OH)4](aq) + H2SO4 (aq) 2 Al(OH)3 (s) + K2SO4 (aq) + 2 H2O (III) 2Al(OH)3 (s) + H2SO4 (aq) Al2(SO4)3 (aq) + 6 H2O (IV) K2SO4 + Al2(SO4)3 + 24 H2O 2 KAl(SO4)2.12H2O The theoretical yield was accumulated over a few
