Archer G11 Partner: Mi 6 Sept Analysis of Alum, AlK(SO 4 ) 2 *12H 2 O

Transcription

1 Analysis of Alum, AlK(SO 4 ) 2 *12H 2 O Purpose: The purpose of this lab is to determine the melting point of alum and the number of water molecules that can be attached to one alum molecule. The significance of this lab is that it could be useful for electronics store. The store can find a substance that can absorb a lot of water and has a high melting point in order to remove the water from the air around the electronics. The reason that the substance should have a high melting point is that the substance would be less efficient at holding water in its liquid stage. Hypothesis: The hypothesis is that the melting point can be found by slowly heating the alum until it melts while the amount of water alum can absorb can be found by massing the alum with water and without water then use stoichiometry on that result. The change in mass before and after heating is the mass of water in the initial mass which can be used to calculate the mole ratio. Materials: Materials Trial 1 Trial 2 Trial 3 Alum (AlK(SO 4 ) 2 ) g g g 50-mL beaker 3 beakers Capillary tubes 3 tubes Bunsen burner 3 burners Wire gauze 6 gauzes Rubber band 3 rubber bands Clay pipe stem triangle 3 triangles One hole rubber stopper 3 stoppers Thermometer 3 thermometers g precision balance 1 balance 30-mL crucible and lid 3 crucibles and lids Ring stand 3 ring stands Ring clamp 3 ring clamps Universal clamp 3 universal clamps Crucible tongs 2 tongs Spatula 1 spatula Timer 3 timers Mortar and pestle 1 mortar and pestle Plier 1 plier Glycerol 3 ml Procedures: Part 1 Melting Point Determination of Alum 1.) Use a spatula to transfer some alum to the mortar 2.) Crush the alum powder using the pestle and mortar 3.) Use a plier to pick up the blue side of the capillary tube

2 4.) Move that side of the tube to the fire 5.) Use the other hand to slowly turn the capillary tube 6.) While turning, pull the capillary tube to close it 7.) Put melted side of the capillary tube in the fire 8.) While the tube is still hot, lightly knock the melted side on the table to make sure that the side is closed 9.) Pack the alum about 0.5cm into the tube 10.) Put a small amount of glycerol on the top of the thermometer 11.) Put the thermometer through a one hole rubber stopper 12.) Use a rubber band to tie the capillary tube and the thermometer together (with the open end of the tube facing down 13.) Pour some water to a 50-mL beaker 14.) Put a wire gauze on the ring clamp on the ring stand 15.) Put the beaker on the wire gauze 16.) Put the rubber stopper (with thermometer) on the universal clamp of the same stand (on top of the ring clamp) 17.) Light a Bunsen burner under the wire gauze 18.) Dip the thermometer and alum into the water 19.) Slowly increase the temperature 20.) As soon as the alum disappear, record the temperature 21.) Repeat step 1-20 for the 2 nd and 3 rd trial Part 2 Determination of the Water of Hydration in Alum Crystals 1.) Put a clay pipe stem triangle on a ring clamp on a ring stand 2.) Put a crucible and its lid on the triangle 3.) Heat the crucible with highest flame intensity for about 5 minutes 4.) Use tongs to move the crucible and lid from the triangle to a wire gaze 5.) Let them cool for 10 minutes 6.) Mass the crucible with its lid 7.) Move the lid off the crucible (while it s on the balance) 8.) Use a spatula to add about 2g of alum crystal to the crucible 9.) Put the lid back on 10.) Mass the crucible, lid, and alum crystal 11.) Use tongs to move the crucible on a triangle 12.) Put the lid on the crucible, leaving a small opening 13.) Gently heat the crucible 14.) When the bubbling stops, wait for 5 minutes longer 15.) Using tongs, move the crucible s lid to a wire gauze 16.) Move the crucible to a wire gauze 17.) Let it cool for 10 minutes 18.) Put the lid on the crucible

3 19.) Mass the crucible, lid, and alum after heating 20.) Repeat step ) Repeat step 1-20 for 2 more trials Results: The alum was a white crystal. The alum lost its brightness after it had been crushed using mortar and pestle. When melted, it turned into a colorless liquid. When the alum from the capillary tube slipped out, it turned into small bits floating around in the water, or floaters. Floaters still melted when it reached its melting point and turned into a transparent liquid. Melting Point Determination Table Trial 1 Trial 2 Trial 3 Measured Melting Point ( C) Literature Melting Point ( C) 92.5 Literature melting point from: Water of Hydration Table Trial 1 Trial 2 Trial 3 Mass of crucible + lid (g) Mass of crucible + lid + alum crystals (g) Calculated mass of alum crystals (g) Mass of crucible + lid + alum after heating #1 (g) Mass of crucible + lid + alum after heating #2 (g) Average mass of crucible + lid + alum after heating (g) Calculated mass of water driven off (g) Calculated mass of anhydrous alum, AlK(SO 4 ) 2 (g) Calculated moles of H 2 O (mole) Calculated moles AlK(SO 4 ) 2 (mole) Calculated mole ratio; moles H 2 O/moles AlK(SO 4 ) 2 58 : 5 73 : : 10 Percent error (%) Analysis: Mass of alum crystals = (Mass of crucible + cover + alum crystals) (Mass of crucible + cover) Mass of alum crystals equals the mass of crucible, lid, and alum crystals subtracting the mass of crucible and lid Trial 1: = g AlK(SO 4 ) 2 Trial 2: = g AlK(SO 4 ) 2

4 Trial 3: = g AlK(SO 4 ) 2 Average mass of crucible + lid + alum after heating = [(Mass of crucible + lid + alum after heating #1) (Mass of crucible + lid + alum after heating #2)] / 2 Average mass of crucible + lid + alum after heating equals to the mass of crucible, lid, and alum after heating #1 minus the mass of crucible, lid, and alum after heating #2 then divide by 2 Trial 1: ( ) / 2 = g Trial 2: ( ) / 2 = g Trial 3: ( ) / 2 = g Mass of water driven off = (Mass of crucible + lid + alum crystal) (Average mass of crucible + lid + alum after heating) Mass of water driven off equals the mass of the crucible, lid, and alum crystal minus the average mass of the crucible, lid, and alum after heating Trial 1: = g H 2 O Trial 2: = g H 2 O Trial 3: = g H 2 O Mass of anhydrous alum = (Mass of alum crystals) (Mass of water driven off) Mass of anhydrous alum equals to mass of alum crystals minus mass of water driven off Trial 1: = g AlK(SO 4 ) 2 Trial 2: = g AlK(SO 4 ) 2 Trial 3: = g AlK(SO 4 ) 2 Mole of H 2 O = (Mass of water driven off) / (Molar mass of water) Mole of H 2 O equals to mass of water driven off divide by the molar mass of water Trial 1: / = mole H 2 O Trial 2: / = mole H 2 O Trial 3: / = mole H 2 O Mole of AlK(SO 4 ) 2 = (Mass of anhydrous alum) / (Molar mass of AlK(SO 4 ) 2 ) Mole of AlK(SO 4 ) 2 can be found by dividing the mass of anhydrous alum the molar mass of AlK(SO 4 ) 2

5 Trial 1: / = mole AlK(SO 4 ) 2 Trial 2: / = mole AlK(SO 4 ) 2 Trial 3: / = mole AlK(SO 4 ) 2 Mole ratio = (moles of the compound) / (moles of the smallest number) * (2, 5 or 10) Mole ratio can be found by dividing the moles of the compound by the moles of the smallest number then multiply by 2, 5 or 10 to get the whole number ratio Trial 1: [( ) / ( )] * 5 = 58moles H 2 O Trial 1: [( ) / ( )] * 5 = 5moles AlK(SO 4 ) 2 Trial 2: [( ) / ( )] * 10 = 73moles H 2 O Trial 2: [( ) / ( )] * 10 = 10moles AlK(SO 4 ) 2 Trial 3: [( ) / ( )] * 10 = 119mole H 2 O Trial 3: [( ) / ( )] * 10 = 10moles AlK(SO 4 ) 2 Percent Error = 100% - (Moles of H 2 O from ratio to 1 mole AlK(SO 4 ) 2 ) / 12 Percent Error equals to 100 percent minus the moles of H 2 O from ratio to 1 mole AlK(SO 4 ) 2 divide by 12 (moles of water that can actually attach to alum) Trial 1: 100 % 11.6 / 12 = 3.33 % Trial 2: 100 % 7.3 / 12 = % Trial 3: 100 % 11.9 / 12 = 0.83 % The hypothesis is confirmed. The average melting point of alum from all trials of part 1 is 92, which is nearly equal to the literature melting point of alum. Not only that, the results of the 2 nd trial part 2 seemed to have some kind of errors in it. However, the results of the 1 st and 3 rd trials part 2 are about 90% accurate. The real ratio of water to alum is 12 to 1 while the results from trial 1 and 3 are about to 1. The object must be cooled before determining its mass because the hot air would cause convection current if there s no lid or, if the lid is on, the hot air would move up and cause the mass to be lighter than it really is. The results on the melting point and amount of water in alum could be used to determine whether the substance is actually alum or not. It can be done by comparing the melting point and amount of water of the substance then compare them to the melting point and amount of water of the alum. There are also other ways to determine whether the substance is alum or not. It could be done by comparing the boiling point of the substance and alum, or the density. It could also be done by reacting the substance and using percent composition and mole ratio to find the empirical formula of the substance and determine whether if the substance is alum or not.

6 Conclusion: The hypothesis is correct from the general results obtained. There are some errors which caused the results to be inaccurate. One of the known errors that was made was that the crucible lid was broke. Even though the piece that broke was put on the scale to increase the mass back to how it was before It broke, not all the mass was added back, thus the mass of the water driven off became less than it was supposed be because it was calculated from a number that was changed by this error. The other error that could have caused the inaccuracy in the results was the time that was used to reheat the crucible with the alum. When it was reheated, there were no bubbles and so it was heated for only five minutes without any signs that water is being removed. Some water could still be in the alum and so it caused that mass of the alum to be higher than it really should be which changed the ratio of the water to alum. In the future, the crucible lid should be handled more carefully in order to not change the mass of it and the reheating of alum should be heated for slightly longer in order to make sure that all the water is removed from the alum.

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