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Preparation and analysis of a complex metal salt

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Experiment 7 Preparation and analysis of a complex metal salt (15-12-2006) Objective 1. To prepare a sample of copper-ammonia complex salt. 2. To determine the ammonia content in the sample of salt. 3. To determine the metal content of the sample of salt. Results A. Determination of ammonium content HCl was added in excess into the complex salt. The NH3 ligands were protonated to NH4+ and were detached from the metal cation. The volume of HCl in excess was determined by titration using NaOH. The ammonium content of the salt was determined. * Weight of the complex salt used: (6.321 - 5.896) = 0.425g * Volume of HCl (0.6M) added onto the flask: 25cm3 No. of moles of HCl added: 25/1000 x 0.6 = 0.015 moles * The reaction of the titration is given by the following chemical equation: HCl + NaOH --> NaCl + H2O (1) Volume of NaOH (0.5M) used in the titration: 20.8 - 4.3 = 16.5 cm3 (2) No. of moles of NaOH used: 16.5 / 1000 x 0.5 = 8.25 x 10-3 moles (3) No. of moles of HCl in excess = No. of moles of NaOH used in titration = 8.25 x 10-3 moles * No. ...read more.


So, the no. of moles of SO42- in the sample of salt is 1.74 x 10-3 mole * Molar mass of SO42- : 32.1 + 16 x 4 = 96.1 g/mole * Mass of SO42- in the sample = 96.1 x 1.74 x 10-3 = 0.167 g. * % by mass of SO42- in the sample = 0.167 / 0.434 x 100% = 38.5% * The metal cation, anion and ligand account for (38.5 + 25.5 + 27.0) = 91% of the weight of the salt. So, the salt consists of 9% of water. C. Determination of the chemical formula of the complex salt * % by mass of NH3 in the sample was 27.0%. * % by mass of Cu2+ in the sample was 25.5% * Assuming 100g of the salt, the no. of moles of NH3 and Cu2+ are (27 / 17) = 1.59 mole and (25.5 /63.5) = 0.40 mole respectively. * The molar ratio of NH3 to Cu2+ is (1.59 / 0.40) = 3.97, which is similar to the theoretical ratio of 4:1. * % by mass of water was 9%. Assuming 100g of the salt, there are 9 g of water in the salt sample. ...read more.


But if there is an excess amount of HCl present, it would be ensured that all the ligands are detached from the metal ion and have reacted with HCl. 2. In the preparation of the complex salt, 95% ethanol was added into the solution. Since the complex salt is less soluble in ethanol then in water, adding ethanol into the flask would make the salt precipitate. 3. After some calculations, it was found that there was 5% of water in the sample of salt. The water comes from 2 sources. First, the water molecules may form the ligands in the complex, as suggested by the chemical formula [Cu(NH3)4(H2O)1.25]SO4. Second, the solution, while being precipitated, may be crystallized and forms water of crystallization. 4. In part B of the experiment, the procedures in part A was repeated but no indicators were added. This is because the indicators would affect the color of the solution, which would in turn affect the color change during the EDTA titration. Thus, no indicator was added. 5. It was found that the ratio of water to Cu in the complex is not an integer. This may be due to the assumption that 'all the weight of the salt, apart from Cu2+, NH3 and SO42-, would be water' being wrong. There may be other substances in the complex salt. The other explanation is that the arrangement of the complex is irregular. ...read more.

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