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Chemistry Lab - Alum.docx

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University of British Columbia
CHEM 121
Steven Barnes

1. Weight aluminum to +/- 0.01g. The initial mass of aluminum is required to calculate the theoretical yield. 2. Add KOH solution into a beaker over a hotplate containing the aluminum pieces under the fume hood. The hotplate will speed up the dissolution process. Stirring the beaker will also help the dissolution process. Do this under the fume hood because Hydrogen gas is produced in the reaction: 2Al(s) + 2KOH(aq) + 6H2O(l)  2Al(OH)4- (aq) + 2K+aq) + 3H2(g). 3. Make sure that the solution is not over 30mL. If the solution is over 30mL, you can evaporate some of the water out using the hotplate (at a later step). 4. Remove any impurities from the solution by vacuum filtration. The filter flask should be extremely clean so that impurities do not alter the concentration or volume of your solution. Make sure you do this step while the solution is still hot so that the impurities don’t cool down and dissolve. It will be more difficult to filter the impurities out of a cool solution as opposed to a hot solution. 5. Transfer the solution to a clear 250mL beaker. Make sure you use less than 10mL deionized water to rinse. Using too much deionized water will dilute the solution and tamper with your results. If you have added too much, you can evaporate some of the water out by heating your beaker and solution over a hotplate (at later step). Cool you solution. 6. Add 20mL of 9M H2SO4 to the cooled solution. The volume still should not exceed 50mL. We add H2SO4(aq) to get the reaction: 2K+(aq) + 2Al(OH)4-(aq) + H2SO4(aq)  2K+(aq) + 2Al(OH)3(s) + 2H2O(l) + SO42-(aq). Make sure you add the sulfuric acid slowly as it can splash up; H2SO4 is highly corrosive. The reason we add sulfuric acid is to produce a white gelatinous precipitate, Al(OH)3. This solution should be clear so that you can dissolve the precipitate. 7. If your solution is over 50mL, you can evaporate some of the water out in this step. Evaporating the water will allow the crystallization to yield more accurate results. 8. In this step, we must encourage crystallization within the solution. Place the solution in the ice bath and scratch the bottom of the beaker with a stirring rod to ensure that crystals develop. Allow adequate time for the crystals to grow (approximately 20 minutes). Crystals form because the solubility of the alum in water decreases with decreasing temperature. Time is a factor in crystallization because the longer the time allowed for crystallization, the slower the crystals grow which results in a pu
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