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Material Type: Lab; Class: Introductory Chemistry I; Subject: Chemistry; University: Laredo Community College; Term: Unknown 1989;
Typology: Lab Reports
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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 CuSO 4 •5H 2 O (used in plant food), MgSO 4 •7H 2 O (Epsom salts), or FeSO 4 •7H 2 O, 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 CuSO 4 •5H 2 O (s) CuSO 4 (s) + 5 H 2 O (g) FeSO 4 •7H 2 O (s) FeSO 4 (s) + 7 H 2 O (g) MgSO 4 •7H 2 O (s) MgSO 4 (s) + 7 H 2 O (g)
Chemicals: Hydrated Salts (CuSO 4 •5H 2 O ( blue ), FeSO 4 •7H 2 O ( green ), MgSO 4 •7H 2 O ( white )), Propane (C 3 H 8 ) Apparatus: evaporating dish, electronic balances, crucible tongs, scoop spatula, tripods, triangles, wire gauze, bunsen burner, sandpaper Safety Equipment: goggles, gloves, hood. Objectives: In this experiment you will learn how to:
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.
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 FeSO 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. % 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% %