Determining the Formula of Hydrate - Lab Report Example

LAB 6 CHEM 1411 Report 2: ________________________________ (0.5 mark) Aim mark) The aim of the experiment is to determine the percentage of barium chloride dehydrate in an unknown sample of sand. Methods (7.5 marks) Materials and apparatus Barium chloride dehydrate, Tongs, Crucible and cover, Clay triangle, Electronic balance, Tripod stand, Burner, unknown mixture Part A: Determining the Formula of hydrate In part, A the sample is hydrated barium chloride. In part, B is a mixture of sand and an unknown amount of hydrated barium chloride. From the sample in part A, we can determine the empirical formula of hydrated barium chloride clearly showing the proportions of the molecules. A clean crucible was placed in a clay triangle and heated strongly for one minute. It was then cooled weighed using an electronic balance and its mass recorded. 3 g of hydrated barium chloride was added to the crucible and then weighed again to find the mass of the crucible with the sample. The crucible was then heated gently on a clay triangle for 10 minutes Cooled and weighed to determine its mass after heating. Heating was continued for 5 minutes until a constant mass was obtained (all the water has been driven off the sample). The crucible was cooled weighed and its mass recorded. The dry barium chloride from the crucible was disposed of, and the crucible washed with water. The crystal structure of the solid may change and may be characterized by a color change. In this experiment, the observation of a constant mass is an indication that we have obtained the anhydrous barium chloride. Part B: Composition of the unknown: The procedure above was repeated. However, instead of 3 g of hydrated barium in the crucible, 3 g of the unknown mixture of dry sand and hydrated barium chloride was put into the crucible. From this composition, the percentage of hydrated barium chloride can be determined in the unknown composition. Precautions (1.5 mark) List 3 lab safety procedures that we followed while performing the experiment After each heating, the crucible was left to cool before measurements were taken Caution was taken that no anhydrous barium chloride evaporated Heating was performed in an open area or fume cupboard in case the evolved gas had the potential risk Results Part (A): (3 marks) Weight of crucible = 25.5754 g Weight of crucible + hydrated barium chloride = 26.3626 g Weight of crucible + anhydrous barium chloride after first heating = 26.2106 g Weight of hydrated barium chloride = (26.3626-25.5754) = 0.7872g Weight of water = (26.3626-26.2106) = 0.152g % of water = (0.152*100)/0.7872 =19.31% RFM water = (2+16) = 18g/mol Moles of water = (0.152/18) =0.008444moles % of BaCl2 = 100-19.31 = 80.69% Determining the ratio of BaCl2 : H2O Molecules BaCl2 H2O % composition 80.69 19.31 Mass of molecule 0.8069*0.7872 = 0 .6352 0.1931*0.7872= 0.15200832 Moles 0.6352/208.3 = 0.00304941 0.15200832/18 = 0.008445 Mole ratio 0.00304941/0.00304941 0.008445/0.00304941 1 2 The composition of a hydrated salt includes both anions (negative ions) and cations (positive ions) surrounded and weakly bonded by water molecules. The hydrate salt has a fixed number of water molecules associated with it, called waters of hydration or water of crystallization. When salt holds waters of hydration, we call it a hydrated salt or a hydrate. From heating of the barium chloride, it was determined the mass of lost water was 0.152g which is also referred to as water of hydration. The percentage of water in the hydrated barium chloride was calculated to be 19.31%, therefore, the % anhydrous barium chloride is 80.69% that corresponds to a mass of 0.6352g. The moles of BaCl2 and water were determined from their masses to be 0.003049 and 0.008445 moles respectively. The mole ratio was determined by dividing each number of moles with the smallest number of moles (i.e. 0.003049) and the mole ratio BaCl2: H2O was found to be 1:2. From the calculation of the empirical formula, it was determined that the ratio of barium chloride to water was 1:2 therefore Barium chloride dehydrate has two waters of crystallization also known as waters of hydration. Part (B): (3 marks) Weight of crucible = 41.0329 g Weight of crucible + unknown before heating = 46.0179 g Weight of crucible + unknown after first heating = 45.6778 g Total mass of sample = (46.0179-41.0329) = 4.985 g Mass of water lost = (46.0179-45.6778) = 0.3401 g Moles of water lost = (0.3401/18) = 0.018 mole Moles of barium chloride in the sample = (0.018/2) = 0.009 mole Mass of barium chloride in the sample = 0.009 × 208 = 1.872g Mass of hydrated barium chloride = (1.872+0.3401) = 2.2121g Molar mass of hydrated barium chloride = 244.3 g/mole % barium chloride hydrate in sample = (2.21/4.985) = 44.38 % From the mass difference, we can determine the mass of water evolved when the unknown mixture was heated. This mass of water is the water of hydration (0.3401g). The moles of water lost were calculated as 0.018 moles. From the empirical formula in Part A the ratio of BaCl2:H2O was found to be 1:2, therefore, the moles of BaCl2 is 0.009moles. The percentage of hydrated barium chloride can be determined directly using the mole ratio to determine the mass to be 2.2121g that corresponds to 44.38%. Therefore, the unknown mixture contains 44.38% hydrated barium chloride Discussion (10.5 marks) What does hydrated barium chloride mean? Describe its physical structure Most solid chemical compounds contain some water in small amounts that may be adsorbed on the surface of the solids whereas other solid compounds contain large quantities of water that are chemically bonded to the crystal. Chemical compounds that contain discrete quantities of water molecules in their crystal structure are called hydrates. The water is bound in definite stoichiometric proportions, and the number of water molecules bound per metal ion is characteristic of the metal ion. What does dehydration mean? Why did we use it in our experiment? Heating a hydrate to temperatures slightly above the boiling point of water removes the water of hydration. Hydrated barium chloride decomposes to water and anhydrous barium chloride when heated to temperatures greater than or equal to 1150C (Slowinski et al., 2012). True hydrates typically undergo reversible dehydration. Therefore, the addition of water to anhydrous barium chloride will result in the formation of barium chloride dehydrate. The amount of water evolved is equivalent to the quantity of water contained in the hydrated barium chloride. If know the value of the water released, we can determine the formula for hydrated barium chloride. Explain what you understand from the molar ratio that you calculated in part (A) using 1-2 sentences The molar ratio represents the proportions of the molecules in the hydrated salt. From it, we can determine how the molecules in the salt are constituted. From the mole ratio, we concluded that hydrated barium chloride consists of 1 part barium chloride and two parts water If you know that after heating the hydrate, the amount of the lost water is 0.44 moles. Calculate the quantity of the hydrated barium chloride (Hint: molar ratio) Moles of water = 0.44moles Mass of water = (0.44× 18) = 7.92g Mole ratio of water: BaCl2 = 2:1 Moles of BaCl2 = (0.44/2) = 0.22moles Mass of BaCl2 = 0.22 × 208 = 45.76g Mass of hydrated barium chloride = 45.76+ 7.92 = 53.68g Write down the dehydration equation for the hydrated barium chloride using the correct molecular formula BaCl22H2O (s )  BaCl2 (s) + 2 H2O (g) Explain what you understand from the percentage that you calculated in part (B) using 1-2 sentences In part B, the percentage barium chloride in an unknown sample was determined. The mixture contained 44.33% barium chloride. Conclusion (1 mark) This experiment aimed to determine the percentage amount of hydrated barium chloride in an unknown mixture of dry sand and hydrated barium chloride. The term hydrate is defined as substances of mineral salts that form crystalline compounds. These hydrates contain water molecules that are distinct from the whole compound, but connected to the structure by bonds. These bonds are weak compared to the bonds that form anhydrous salt and the bonds that form water molecules. For instance, MgSO4 is an example of common hydrate salt. Furthermore, when hydrates are heated they are converted to the anhydrous form by removal of the water of hydration. The anhydrous salt is defined as the compound without water. Information about the chemical formula of the hydrate can be provided from the process of dehydrating. So, to find the mass of water loss, the hydrate compound is weighed before and after dehydrating. Moreover, from that, the moles of water released by dividing the mass by the molar mass. Moreover, to know the moles of water per moles of the hydrate compound, the molar ratio of water released to moles of dehydrate compound must first be determined. When heating hydrated BaCl2, anhydrous barium chloride, BaCl2, and H2O are formed. They form after weighing the mass after heating for both the hydrated barium chloride and the unknown mixture, I have to wait for five minutes at least to let it cool except for anhydrous salt, it must be measured immediately because if not it will evaporate. References Slowinski, Emil J., Wayne C. Wolsey, and Robert C. Rossi (2012). Chemical Principles in the Laboratory. Australia: Brooks/Cole. Read More