AP Chemistry: Analysis Of Alum

AP Chemistry
Analysis of Alum

* Purpose: * In this lab we performed several tests to determine if our crystals were actually aluminum potassium sulfate. * Procedures: * Materials: * Chemicals: * Aluminum potassium sulfate, 2.5 g * Equipment – Part 1: * 150 mL beaker * Bunsen burner * 2 capillary tubes * Mortar and pestle * Notched stopper to hold thermometer * Ring stand * Rubber band * Thermometer * Universal clamp * Equipment – Part 2: * Balance, .001 g precision * Bunsen burner * Crucible and cover *
Crucible tongs * Ring stand, ring support * Triangle * Wire gauze * Procedure - Part 1. Melting Point Determination of Alum: * Pulverize 0.5 g of dry alum. * Pack alum in capillary tube. Push open end of capillary tube into pile of alum. * Turn tube so the open end is up, and bounce the bottom of the tube on the desktop several times. * Fasten tube to thermometer with rubber band. * Use thermometer clamp and notched cork stopper to fasten the thermometer to ring stand. * Immerse bottom of capillary and thermometer in beaker of water and heat. * Record temperature range at which alum melts in data table. * Repeat melting point determination, using new sample of alum and new capillary tube.

* Procedure – Part 2. Determination of the Water of Hydration in Alum Crystals * Set up Bunsen burner on ring stand beneath a ring clamp holding a clay pipe stem triangle. * Adjust height of ring clamp so that bottom of crucible sitting in clay triangle is 1 cm above burner. * Place crucible with cover in clay triangle and heat over burner flame until crucible is red hot. * Turn off gas and remove burner. * Using crucible tongs, remove crucible cover and place it on wire gauze on tabletop.
Using tongs, remove crucible from clay triangle and place it on wire gauze. * Allow crucible to cool for 10 minutes. * Use analytical balance to find mass. Handle with tongs to avoid getting finger prints on crucible and lid. * Record mass in data table. * Add 2 g of alum crystals to crucible. Weigh crucible, cover, and crystals and record mass in data table. * Set crucible at angle in triangle held in ring on ring stand. Cover crucible loosely with crucible cover, and heat gently. Alum will melt, and water of hydration will evaporate. * After bubbling has stopped, heat sample more strongly for five minutes. * Turn off gas and remove burner. * Using tongs, remove crucible cover and place it on wire gauze on tabletop. With tongs, remove crucible from clay triangle and place it on wire gauze. * Allow crucible to cool for 10 minutes. * Measure and record mass of crucible, cover, and anhydrous alum. * Repeat procedure until constant mass is obtained. * Record final mass of crucible, cover, and anhydrous alum in data
table. * Dispose of anhydrous alum. Clean crucible and cover. * Procedure – Part 4. Flame test for the K+ Ion * Crush small sample of alum crystal. * Heat flame loop red hot in Bunsen burner flame and then plunge loop in 0.5 M HCl. Repeat. Heat loop to red hot one more time. * Pick up small sample of crushed alum on the hot loop and move it slowly from outer edge to the hottest part of the Bunsen burner flame. * Observe and record the resulting color of the flame test. * Data Collection: * Part 1 Data Table: | Trial #1 | Measured melting point | 92.5 ℃ | Literature melting point | 92.5 ℃ |

* Part 2 Data Table: Mass of crucible and cover | 17.2524 g | Mass of crucible, cover, and alum crystals | 19.3267 g | Mass of alum crystals | 2.0743 g | Mass of crucible, cover, and alum after heating | 18.6561 g | Mass of water driven off | 0.6706 g | Mass of anhydrous alum, AlK(SO4)2 | 1.4037 g | Moles H2O | .03722 mol | Moles AlK(SO4)2 | .0054361 mol | Mole Ratio; moles H2O/moles AlK(SO4)2 | 6.847 mol |

* Part 4 Data: * The flame turned a violet color. IV) Data Analysis: * Pre-Lab Questions: * When measuring a melting point, why is it necessary to raise the temperature very slowly in the vicinity of the melting temperature? * If you raise the temperature too quickly you might miss when the compound starts to turn to liquid, so you would miss the initial reading and you would probably get an incorrect melting point. * Washing soda is a hydrated compound whose formula can be written Na2CO3∙xH2O, where x is the number of moles of H2O per mole of Na2CO3. When a 2.123 g sample of washing soda was heated at 130℃, all of the water of hydration was lost, leaving 0.787 g of anhydrous sodium carbonate. Calculate the value of x. * 2.123 g-0.787 g=1.336 g H2O * 1.336 g H2O × 1 mol H2O 18.016 g H2O =0.07416 mol H2O * 0.787 g Na2CO3 × 1 mol Na2CO3 105.99 g Na2CO3= .00743 mol Na2CO3 * 0.00743 mol Na2CO30.00743=1 mol Na2CO3 * 0.07416 mol H2O 0.00743=9.98 ≈10 mol H2O * Na2CO3∙10H2O, so x =10 * Post-Lab Calculations and Analysis * Part 1: * Find the literature value for the melting point of aluminum potassium sulfate and enter this value in the Part 1 Data Table. (a) 92.5℃ * Part 2: * From the mass of anhydrous alum remaining in the crucible after heating and its formula, AlK(SO4)2, calculate the moles of anhydrous alum in the original sample. Enter this value in the Data Table. (b) 1.4037 g AlK(SO4)2 × 1 mol AlK(SO4)2 258.22 g AlK(SO4)2 = .0054361 mol AlK(SO4)2 * From the mass of water driven off from the sample and the molar mass of water, calculate the moles of water in the original sample. Enter this value in the Data Table. (c) .6706 g H2O × 1 mol H2O 18.016 g H2O = .03722 mol H2O * Calculate the mole ratio of water to anhydrous alum in the sample. Record this value in the Data Table. (d) .03722 mol H2O .0054361 mol AlK(SO4)2=6.847 mol * Post-Lab Questions: * Why must objects be cooled before their mass is determined on a sensitive balance?
* Placing a hot object on a sensitive balance would cause convention currents in the air around the object. This would lead the reading on the balance to be unsteady. Also, when the object is hot it has more energy, so it would lead to having a bigger mass. * Comment on the results of the different tests used to verify that the sample tested was alum. * The first test was to determine the melting point of alum. The measured melting point that we got was 92.5 ℃. The literature melting point of alum is also 92.5℃. Also, during our flame test the flame turned purple which proved the presence of potassium ions. * What other tests could be used to verify the composition of alum? * You could perform a sulfate test to determine the percentage of sulfate that was present. You could also determine the percentage of aluminum or potassium in the crystals. V)
Conclusion: * In this lab we performed three tests to see if our crystals were really alum. We were able to conclude that our crystals were alum crystals by the tests that we performed. We found the melting point of our crystal to be 92.5 ℃. The literature melting point for alum is also 92.5 ℃, so we can conclude that we did make alum. Also, from our flame test we were able to conclude that K+ ions were present because the flame turned violet. From our second test, determining the water of hydration in the alum crystals, we found the mole ratio to be 6.847 moles. This was our only test that had a major error; the accepted mole ration is 12 moles. Some relevant chemical concepts include melting point, water of hydration, and precision. We had to calculate the melting point of alum for our first test. We had to find the number of moles of water of hydration in the formula for our second experiment. We also had to be very precise with finding the masses. In summary, there are several tests that one can perform to see if the compound he or she created was actually the compound that he or she wanted to create. VI) Complete Error Analysis: * One major error in this lab would be to not remove all the water from the salt. If you did not remove all of the water, this would lead to you having a lower mass of water driven off, which means you would have less moles of water; you would also have a lower mole ratio, which would make your percent error very large. Another possible error would be touching the crucible. This would leave finger prints on the crucible and you would then have a larger mass of alum. You would then have more moles of alum, which would lead to a smaller mole ratio. We had one major error in our lab; we found our mole ratio to be 6.847 moles when it should have been 12 moles. I think our error resulted from an error in measuring the 2 grams of alum crystals. I think we added too much alum to the crucible. The larger mass of alum crystals led to a larger amount of alum remaining after heating. This led to a smaller mole ratio and a larger percent error. * Percent Error: * |Actual yield - theoretical yield| theoretical yield ×100=% Error * |6.847 mol -12 mol| 12 mol ×100=43% Error