OBJECTIVES: EXPERIMENT #5 Water of Hydration Observe changes in color and form of a hydrate after heating strongly Accurately weigh reaction mixtures before and after heating Calculate the mass of water driven off from the hydrate Calculate the percentage of water in the hydrate Determine an unknown hydrate from a list of possibilities BACKGROUND: Many ionic solids can occur in a form in which the crystals contain water chemically bound to the ions forming the crystalline lattice. This water is known as water of hydration or water of crystallization, and the solid is known as a hydrate. These same ions can also occur in anhydrous form that is, without water of hydration. Addition of water to the anhydrous residue affords the hydrate. Some common hydrates are listed below: alum K2SO4. Al2(SO4)3. 24H2O potassium aluminum sulfate blue vitriol CuSO4. 5H2O copper sulfate borax Na2B4O7. 10H2O sodium tetraborate washing soda Na2CO3. 10H2O sodium carbonate When copper(ii) chloride dihydrate is heated above the boiling point of water, the crystalline structure of the salt changes and, in this case, there is also a color change from green crystals to a brownish yellow powder. hydrated form CuCl2. 2H2O(s) green crystals heat anhydrous form CuCl2(s) + 2H2O(g) brownish yellow Cobalt(II) chloride forms hydrates which are used in inexpensive hygrometers -- a device used to indicate the relative humidity of air. moist air red violet blue CoCl2. 6H2O 4H 2 O CoCl2. 2H2O O 2H 2 CoCl2 Hydrates have other properties of practical importance. The hardening of cement (a complex mixture of limestone and clay containing materials) is due, in part, to the formation of hydrates. (See figure on page 34.) Plaster-of-Paris, a hydrate of calcium sulfate, is formed when gypsum, another hydrate of calcium sulfate, is allowed to react with water. Plaster-of-Paris expands as it hardens, thus finding applications in surgical casts and plastering walls. (CaSO4)2. H2O + 3H2O 2CaSO4. 2H2O gypsum plaster-of-paris dry air 43 P a g e
EXPERIMENT #5 WATER OF HYDRATION If we are given the formula of a hydrate, we can calculate the percentage water of hydration. We do this the same way we would calculate percentage composition. For example, the molar mass of copper(ii) chloride dihydrate is 1Cu = 1(63.55) = 63.55 2Cl = 2(35.45) = 70.90 4H = 4(1.008) = 4.032 2O = 2(16.00) = 32.00 = 2H2O = 2(18.02) = 36.04 = 170.48 g/mol The percentage water is found by dividing the part -- the mass of water in one mole of the hydrate -- by the whole -- the molar mass of the hydrate. 36.04 2 %H 2O x10 21.14% 170.48 If we are given an unknown hydrate, we can calculate the percentage water of hydration experimentally. A sample of the hydrate of known mass is heated until all the water of hydration is driven off. The mass of water lost is equal to the difference in masses between the hydrate sample and the residue. The percentage water can be calculated as follows: Example: mass of water lost %H 2O x10 mass of hydrate sample After heating a 1.324 g sample of a hydrate a residue weighing 0.846 g is observed. the mass of hydrate is: 1.324 g the mass of anhydrous residue is: 0.846 g the mass of water lost is: 0.478 g mass of water lost 2 %H 2O x10 mass of hydratesample 2 g 0.478 x10 2 36.1% 1.324g 44 P a g e
EXPERIMENT #5 WATER OF HYDRATION EXPERIMENTAL PROCEDURE: (Work individually. Extinguish the Bunsen flame when not in use.) 1. Obtain a sample of hydrate from your instructor. Record the identifying letter or number of your unknown on you data sheet. 2. Obtain a clean crucible and properly fitting cover. If necessary, use a brush and soap to clean the crucible. If your crucible cannot be cleaned in this manner, check with you instructor. 3. If your crucible is wet, it can be dried by heating using the apparatus described by you lab instructor. (See FIGURE I below.) 4. Weigh crucible to the nearest thousandth of a gram (that's the third decimal place) and record on your data sheet. Use crucible tongs when carrying crucible from your bench to the balance and back again. 5. Transfer approximately 2 g of your unknown (weighed to the nearest thousandth of a gram) into the crucible, and weigh the crucible and contents on the balance. 6. Set up the apparatus as described by your lab instructor. (See Figure below.) Place the crucible in the clay triangle, set the cover on the crucible slightly ajar, and heat the crucible with a Bunsen burner. 7. Heat the crucible gently for 5 min by waving the flame of the burner across the bottom. Then strongly heat the bottom of the crucible until it glows a dull red for 10 min. Prepare a second sample while the first is undergoing heating/cooling. 8. Turn off the flame. Place the crucible and cover on a wire gauze on the lab bench and allow to cool to room temperature. 9. Weight the crucible and its contents. Record the mass on your data sheet. Dispose of the residue as described by your instructor. 10. Perform a second trial. 11. Optional: Complete the table on page 40 by calculating the percentage of water of hydration. Based on your experimental data, decide which hydrate is your unknown. EXPLAIN YOUR CHOICE. 45 P a g e
EXPERIMENT #5 WATER OF HYDRATION FIGURE I: The crucible cover is left open just enough to let water vapor escape. 46 P a g e
NAME Section Date DATA AND CALCULATIONS: Water of Hydration Letter/Number of unknown sample: Appearance of hydrate before heating Appearance of anhydrous residue Trial 1 Trial 2 Mass of crucible and hydrate, g Mass of crucible, g Mass of hydrate, g Mass of crucible and anhydrous material, g Mass of anhydrous material, g Mass of H2O lost, g mass of water lost 2 10 mass of hydrate sample 2 %H O x Average % water in hydrate _ CLEARLY SHOW CALCULATIONS BELOW: 47 P a g e
OPTIONAL Based on your experimental results and the list below, determine which hydrate is your unknown. Formula BaCl2. 2H2O Na2MoO4. 2H2O % Water of Hydration K2CO3. 3 H2O 2 CaSO4. 2H2O Na2SO4. 10H2O Na2SO4. 7H2O Na2CrO4. 4H2O CaCl2. 2H2O Na2CO3. 10H2O Na2B4O7. 10H2O MgSO4. 7H2O CuSO4. 5H2O ZnSO4. 7H2O Identity of unknown hydrate: _ EXPLANATION: 48 P a g e
NAME Section Date ADDITIONAL ASSIGNMENT I: Water of Hydration The mass of a crucible and a hydrated salt was found to be 21.447 g. The mass of the crucible and the anhydrous salt was 20.070 g. The mass of the empty crucible was 17.985 g. 1. Calculate the mass of the hydrate sample. 2. Calculate the mass of the anhydrous sample. g 3. Calculate the mass of water lost. g 4. Calculate the percent water in the hydrate. g % 49 P a g e
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NAME Section Date ADDITIONAL ASSIGNMENT II: Water of Hydration An unknown hydrate may be one of the three hydrates listed in the table below. After heating a 2.752 g sample of unknown hydrate, 1.941 g of sample remained. Calculate the percent water in the unknown. Calculate the missing items in the table. What is the probable identity of the unknown? EXPLAIN! Hydrate Molar Mass grams H2O per mole Percent Water LiNO3. 3H2O Ca(NO3)2. 4H2O Sr(NO3)2. 4H2O Unknown, 2.752 g 51 P a g e
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