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LAREDO COMMUNITY COLLEGE Science Department CHEM 1411 Laboratory 1 CHEMICAL REACTIONS: Water of Hydration A. Short Overview Many common substances exist as solid ionic salts that contain water molecules embedded in thier crystal structures. This water is referred to as water of hydration, and can be removed from the crystals by heating, or dehydration. Such water commonly gives the hydrated salts different properties, such as color, than the anhydrous form. This is especially obvious when the metal cation is a member of the transition metals. In the first of four quantitative experiments involving chemical reactions (water of hydration, empirical formulas, limiting reagents, and heat of reaction) that you will perform this semester, you will determine the percent water present in an ionic salt. While this is an experiment some of you may have already done in a high school chemistry class, it is a valuable experiment because it reinforces the concept of percent composition. In this experiment you will be provided with a sample of CuSO4•5H2O (used in plant food), MgSO4•7H2O (Epsom salts), or FeSO4•7H2O, and asked to determine the experimental % water and the actual composition of the solid based upon the amount of water removed from the sample. Each sample can be identified by its color. The chemical equations for the reactions are CuSO4•5H2O (s) CuSO4 (s) + 5 H2O (g) FeSO4•7H2O (s) FeSO4 (s) + 7 H2O (g) MgSO4•7H2O (s) MgSO4 (s) + 7 H2O (g) MnCl2•4H2O (s) MnCl2 (s) + 4 H2O (g) B. Procedure Exercise 1. Percent Composition of a Hydrated Salt Chemicals: Hydrated Salts (CuSO4•5H2O (blue), FeSO4•7H2O (green), MgSO4•7H2O (white), MnCl2•4H2O (pink)), Propane (C3H8) Apparatus: electronic balances, crucibles, crucible tongs, scoop spatula, tripods, triangles, wire gauze, bunsen burner Safety Equipment: goggles, gloves, hood. Objectives: In this experiment you will learn how to: 1. heat a sample in a crucible with a Bunsen burner. 2. use the method of “weight-by-difference” to determine mass quantities. 3. convert grams to moles. CHEM 1411 Lab Water of Hydration 2 4. determine the percent of water lost by heating a hydrated sample. Your instructor will assign you a hydrated salt. Each student will heat one sample. On the data sheet, the data for your sample is placed in column listed “Trial 1”. Your partner’s data is listed in column(s) “Trial 2” (and “Trial 3” if a group of three). Use the same electronic balance for each weighing. Tare the balance each time to 0.000 g before weighing the evaporating dish. CAUTION: Tie your hair behind your head. 1. Obtain a Bunsen burner and tripod from the front of the lab. Place a wire triangle from your desk on top of the tripod. 2. Clean your crucible & cover with soap and water, rinse with distilled water, and dry the items with paper towel. 3. Use your crucible tongs to place the crucible and cover on the triangle. (Your instructor will demonstrate this process for you.) 4. Connect the Bunsen burner to the propane gas line with rubber hose. (CAUTION: Check the burner hoses carefully before lighting. Replace the burner hoses if they are old or cracked.) Turn on the gas and light the burner. Adjust the flame so that it is pale blue and does not appear yellow. (Yellow flames are caused by incomplete combustion and will leave soot on the crucible; yellow flames are also cooler than blue flames.) Place the burner under the crucible. 5. Heat the crucible until it is red-hot on the bottom for about 5 minutes. Use the tongs to place the crucible and cover on the desk, and allow it to cool to room temperature. (Caution: The crucible (and cover) will get very hot when heated. Do not touch the items with bare hands.) Allow the crucible and cover to cool to room temperatures before weighing. The items are cool enough to weigh when they feel cool as you pass your hand over them. Do not try to cool down the crucible or cover by running cold water over them; the porcelain material will crack. Also, do not set the hot items on your lab notebooks; they will burn through the notebook paper. 6. Weigh the cool crucible, with lid, on the electronic balance to the nearest 0.001 g. Then use a spatula to add 2.5 – 3.0 grams of the assigned hydrated salt to the dish. Reweigh the crucible and lid to ± 0.001 gram. 7. Replace the wire triangle with the wire gauze and place the crucible and lid on the wire gauze. Heat the crucible gently for five minutes by placing the burner under the wire gauze. 8. Replace the wire gauze now with the wire triangle and heat the crucible strongly at red heat for an additional five minutes. Cool the crucible and lid to room temperature and weigh the items. They should weigh less than they did in step 7 due to the loss of water. 9. Reheat the crucible and lid on the wire triangle with a strong flame for another five minutes, cool and reweigh. The weight after the two heating/cooling increments should agree within 0.005 g. If CHEM 1411 Lab Water of Hydration 3 not, repeat the heating process for three additional minutes, followed by cooling and weighing. 10. Clean the crucible and lid by dissolving the solid in tap water. Rinse the solution in the sink. Repeat the cleaning process of the crucible and lid with soap and tap water. At the end rinse the dish and lid with distilled water. Dry the crucible and lid with paper towel, and return them to your desk. Return all borrowed equipment to the equipment cart at the front of the lab. CAUTION: Be sure to turn the propane gas off when you finish. D. Disposal There are no special disposal instructions for the experiment. All solutions may be discarded in the sink. E. Calculations 1. Determine the moles of salt in the sample by dividing the mass of the dehydrated salt by the formula weight. 2. Determine the moles of water by dividing the mass of water lost by 18.015. 3. Determine the percent water in the sample by dividing the mass of water lost by the mass of the hydrated salt and multiplying by 100%. E. Lab Report Spreadsheet Open the spreadsheet "Water of Hydration" on the CHEM 1411 website. The URL for the website is http://www.laredo.edu/science. When you open the website, click on the link for "CHEM 1411". You will then click on the icon for the spreadsheet. Enter your data in the cells WITH BORDERS only. As you enter your values, the program will automatically calculate the masses, moles, and % composition. When you finish, print out the data and include in your lab report. A sample spreadsheet is provided on the next page. Questions 1. Determine the experimental % H2O for your samples. 2. Determine the % H2O for the following empirical formulas: CuSO4•5H2O, FeSO4•7H2O, MgSO4•7H2O, and MnCl2•4H2O. 3. It is possible that the experimental % H2O (question 1) may be significantly lower than the calculated theoretical % H2O (question 2) for one of your samples. Provide a possible explanation for this. CHEM 1411 Lab Water of Hydration 4 Trial 1 Trial 2 Trial 3 Unknown number 18 48 27 Mass of empty crucible & cover 18.245 21.882 18.625 g Mass of crucible, cover, and hydrated salt 19.965 23.954 20.593 g Mass of hydrated salt 1.720 2.072 1.968 g Mass of crucible, cover, & dehydrated salt, final heating 19.324 22.907 19.770 g Mass of dehydrated salt 1.079 1.025 1.145 g Mass of water lost 0.641 1.047 0.823 g Moles of water lost 0.035581 0.058118 0.045684 mol Formula of dehydrated salt CuSO4 MgSO4 FeSO4 Molar mass of dehydrated salt 159.6 120.37 151.94 g/mol Moles of dehydrated salt 0.00676 0.00852 0.00754 mol Moles water ÷ Moles dehydrated salt 5.26 6.83 6.06 % water (experimental) 37.3% 50.5% 41.8% % % water in hydrated salt (theoretical) 36.1% 51.2% 45.4% % error 3.3% 1.2% 7.8% % LAREDO COMMUNITY COLLEGE Science Department CHEM 1411 Laboratory 5 Water of Hydration DATA SHEET Name: Date Partner(s) Section Trial 1 Trial 2 Trial 3 Unknown number ( ) ( ) ( ) Color of unknown ( ) ( ) ( ) 1. Mass of empty crucible & cover g g g 2. Mass of crucible, cover, and g g g hydrated sample 3. Mass of sample g g g Mass of crucible & dehydrated sample 4. 1st heating g g g 5. 2nd heating g g g 6. 3rd heating (if necessary) g g g 7. Mass of dehydrated sample g g g 8. Formula of dehydrated salt ( ) ( ) ( ) 9. Molar mass of dehydrated salt g/mol g/mol g/mol 10. Moles of salt mol mol mol 11. Mass of water lost g g g 12. Moles of water mol mol mol 13. Moles water ÷ Moles salt ______________ ______________ ______________ 14. Empirical Formula, hydrated salt ______________ ______________ ______________ 15. Experimental % H2O ______________ ______________ ______________ 16. Theoretical % H2O ______________ ______________ ______________ 17. % Error ______________ ______________ ______________ Instructor’s Approval Show all calculations on the back of the page.
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