11. BE SURE YOU ARE WEARING YOUR SAFETY GOGGLES. Slowly and carefully, stir in 20 mL of 9M sulfuric acid. Initially you will notice the formation of a white, gelatinous aluminum hydroxide precipitate. As you add more of the sulfuric acid, you will notice the evolution of heat and the dissolving of the precipitate. If the reaction goes slowly, warm GENTLY while stirring to complete the reaction.
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12. Evaporate the solution to a volume of about 60 mL using a hot plate. If any solid remains, filter the mixture by water suction and save the clear filtrate for the next step. 13. Prepare an ice bath by filling a 600-mL beaker half-full with crushed ice and just covering the ice with water. Alternatively, use a small metal pan filled with an ice-water mixture. Set the beaker containing the clear filtrate from the last step into the ice-water mixture to cool. Alum crystals should form after about 15 minutes. Good crystals take time to form, so don’t hurry. 14. While the mixture is cooling, clean and reassemble the suction filtration apparatus. When you have a good crop of alum crystals, filter them from the cold solution, transferring as much of the solid product as possible to the funnel. 15. Mix 10 mL of ethanol (ethyl alcohol) and 5 mL of water. Chill this mixture in the ice bath for a few minutes. Using an eyedropper, rinse the remaining crystals from the beaker into the funnel. Rinse the crystals in the funnel with drops of the alcohol-water solution. Allow the crystals to dry for five minutes on the filter paper with full suction. 16. After the crystals are dry, weigh them on the lab balance by the following procedure. Weigh a clean beaker on the balance, then place the crystals in the beaker and weigh the beaker plus crystals. 17. From the mass of aluminum originally recorded, calculate the theoretical yield of alum. 18. From the mass of product recovered and the calculated theoretical yield of alum, calculate your percent yield. 19. Obtain your instructor's approval of your alum crystals. If your instructor wishes to grow a large crystal, turn in your alum for use as the starting material. If you do not use your alum in this way, dispose of it in the waste container provided.