Experiment 8 - Analysis of a Hydrate
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Experiment 8 – Analysis of a Hydrate Name __________________ Lab Section __________________ Experiment 8 – Analysis of a Hydrate Introduction When most people think of a solid substance, it almost never crosses their minds that it could be anything but completely dry (on the inside, that is). However, there are many solids that naturally incorporate water into their solid matrix when they form.. These compounds are called hydrates and are hygroscopic, or water absorbing. Each of these hydrates contains a certain number of water molecules, and it follows the law of constant composition, as do all compounds. The way the waters are notated, however, looks slightly differently. For instance, copper (II) sulfate, CuSO4, incorporates 5 waters when it forms a solid. Its chemical formula is written as CuSO4·5H2O, where the “dot” refers to the fact that the water is not part of the chemical structure, but merely absorbed in the solid. These hydrates can be heated, and the water driven off from the solid. In this case the anhydrous, or “without water”, dry, form of the solid is obtained. Alum is the general name for potassium aluminum sulfate dodecahydrate, or simply, KAl(SO4)2·12H2O. It is used commonly in deodorants and as a styptic pencil to stop bleeding from minor cuts. When it is heated, and dried, it follows the chemical equation: ο πΎπ΄π(ππ4 )2 · 12π»2 π (π ) → πΎπ΄π(ππ4 )2 (π ) + 12 π»2 π (π) In this way, the percentage of water in a sample can be found. Theoretically, the percentage in a hydrate is merely the molar mass of water divided by the molar mass of the compound. Experimentally the amount of water in any hydrate can be determined by comparing the mass of the hydrate before and after heating. In this experiment you will determine, experimentally, the percentage of water in alum and compare that to the theoretical value. After this you will experimentally determine the percentage of water in an unknown hydrate, as well as the water of crystallization: the number of waters in the hydrate. Equipment 100 or 250 mL beaker Watchglass Bunsen burner Wire gauze Iron ring NaHCO3 Unknown sample Experiment 8 – Analysis of a Hydrate Name __________________ Lab Section __________________ Examples 1) Calculate the theoretical percentage of water in alum, KAl(SO4)2·12H2O. First calculate the molar mass of water and of the hydrate as a whole. Alum 1 K: Water π π π π π π 1 x 39.10 πππ = 39.10 πππ 1 Al: 1 x 26.98 πππ = 26.98 πππ 2 S: 8 O: π π 24 H: 24 x 1.01 πππ = 24.24 πππ π π 12 O: 12 x 16.00 πππ = 192.00 πππ 2 x 32.07 πππ = 64.14 πππ ___________ 8 x 16.00 216.24 πππ π πππ π π = 128.00 πππ ____________ π 258.22 πππ π Total mass of alum = 474.46 πππ Calculate the percentage of water in the hydrate: πππ π ππ π€ππ‘ππ % π»2 π = πππ π ππ βπ¦ππππ‘π π₯ 100% π πππ % π»2 π = π π₯ 100% = ππ. ππ% π―π πΆ 474.46 πππ 216.24 2) An unknown hydrate is weighed out, heated, and reweighed after drying. If the original hydrate weighed 1.725 g, but after heating it weighed 0.941 g, what is the percentage of water in the unknown compound? Using Conservation of Matter, the amount of water that is lost is the difference between before and after heating 1.725 g hydrate – 0.941 g anhydrous = 0.748 g water lost. 0.745 π % π»2 π = π₯ 100% = 43.188% π»2 π → ππ. π% π―π πΆ 1.725 π Experiment 8 – Analysis of a Hydrate Name __________________ Lab Section __________________ π 3) If an anhydrous compound has a molar mass of 245 πππ and it was found that the % H2O was 30.6%, calculate the water of crystallization of the compound. Here it is useful to remember that percentages do not depend on the amount of the sample. If you assume that there were 100g of the original sample (even if it were less), then the percentages become masses (30.6% of 100g is 30.6 g). The result is 100 g of hydrate minus 30.6 g water yields 69.4 g anhydrous compound. Taking the masses of the compounds and dividing by the molar masses of the compounds gives the number of moles. These, in turn, can be turned into the ratio of water to anhydrous compound as follows: 1 πππ π» π 30.6 g π»2 O [18.02 π π»2 π] = 1.70 πππ π»2 π 2 1 πππ ππβπ¦ππππ’π 69.4 g anhydrous [ 245 π ππβπ¦ππππ’π ] = 0.283 πππ ππβπ¦ππππ’π 1.70 πππ π»2 π 0.283 πππ ππβπ¦ππππ’π = 6.01 ≈ 6 Therefore, the water of crystallization is a 6:1 ratio, water:anhydrous. The formula of the hydrate is: Anhydrous Compound·6H2O Figure 8.1: Experimental Set Up Experiment 8 – Analysis of a Hydrate Name __________________ Lab Section __________________ Procedure Percentage of Water in Alum Weigh between 0.8 – 1.2 g of alum into a clean, dry 250 mL beaker. Place the watchglass on the beaker. This will act as the indicator of when the experiment is done. Place the beaker on the wire gauze and heat the alum. A vigorous flame will be used in this experiment. Be sure that the tip of the cone of the flame is touching the wire gauze. See Figure 8.1. Water droplets should begin to form on the watchglass within a few minutes. Note: the initial “fog” one sees upon heating is merely the water vapor that is naturally occurring on the glass being driven off by the heat. Continue to heat the alum until the water droplets on the watchglass have disappeared. Allow the beaker to cool to room temperature and weigh it. anhydrous alum resulting from the decomposition. This is the mass of the Calculate the percent of water of the alum from the original mass of the alum. Percent of water in an Unknown Hydrate Weigh out approximately 1 to 2 g of the unknown sample into your beaker. Repeat the steps of the previous procedure. Calculate the average percentage of water in the unknown hydrate. Calculate the water of crystallization of the compound and report its chemical formula. Post Experiment Instructions All waste can be placed in the trash or rinsed down the sink. Return unknown vial to its place of origin. Experiment 8 – Analysis of a Hydrate Name __________________ Lab Section __________________ Prelaboratory Questions 1) What is meant by the following terms: Hydrate Anhydrous Water of crystallization 2) What is the %H2O in a sample of CaCl2·2H2O? 3) An unknown sample of a hydrate was dehydrated and found to contain 13.7% water. If the π molar mass of the anhydrous compound is 227 πππ what is the chemical formula of the hydrate? Experiment 8 – Analysis of a Hydrate Name __________________ Lab Section __________________ Data Table Percent of H2O in KAl(SO4)2·12H2O Trial 1 Trial 2 Mass of KAl(SO4)2·12H2O (before heating) __________ __________ Mass of KAl(SO4)2·12H2O (after heating) __________ __________ Mass of H2O __________ __________ Percentage of water in the hydrate – show calculations Percent of H2O in KAl(SO4)2·12H2O __________ Average percent H2O in KAl(SO4)2·12H2O __________ __________ Experiment 8 – Analysis of a Hydrate Name __________________ Lab Section __________________ Percent of Water in an Unknown Hydrate Unknown Number ____ Trial 1 Trial 2 Mass of Unknown Hydrate Before Heating __________ __________ Mass of Unknown Hydrate After Heating __________ __________ Mass of H2O __________ __________ Percentage of H2O in the Unknown Hydrate – show calculations % H2O in Unknown Hydrate __________ __________ Average %H2O in the Unknown Hydrate __________ π Water of Crystallization of the Unknown Hydrate – Molar Mass Unknown _____________πππ Water of crystallization __________ Chemical Formula of the Hydrate ____________ Experiment 8 – Analysis of a Hydrate Name __________________ Lab Section __________________ Postlaboratory Questions 1) Calculate the percentage of water in the following compounds CoCl2·6H2O CuSO4·5H2O 2) An unknown hydrate sample has a mass of 1.026 g. It is heated until it is dry and it weighs 0.656 g after heating. What is the %H2O of the sample? 3) If the molar mass of the anhydrous compound in problem 2 were 320 g/mol, what is the water of crystallization, and chemical formula of the hydrate? Experiment 8 – Analysis of a Hydrate Name __________________ Lab Section __________________ 4) A sample of a hydrate of CoCl2 is found to have 13.88%H2O. What is the water of crystallization and chemical formula for this hydrate? 5) Gypsum is a naturally occurring hydrate of CaSO4. If it a sample is dehydrated and it is found to contain 21.0% water, what is the chemical formula for gypsum? 6) Alternatively to the above, Plaster of Paris, the mortar like compound used extensively in artwork such as frescos, is a fractional hydrate. This means that its overall chemical formula is not a whole number. If a 1.401 g sample of Plaster of Paris is dried and its resulting mass is 1.315 g, what is the formula for Plaster of Paris?
