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Chm130- Gravimetric Full Report

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Chm130- Gravimetric Full Report
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  1 I. Introduction The evolution of scientific knowledge owes its pace to the intelligence of mankind that  paved techniques to be passed to one generation after another, from the discovery of the nature of atom, to Mendeleev’s creation of the periodic table , leading to T.W. Richards and his students’  determination of the atomic masses of certain elements. The nature of Analytical Chemistry is the identification of composition of materials, either through quantitative analysis  or qualitative analysis . The latter involves a method called Gravimetric Analysis  that plays significant role in the determination of the amount of species in a material through the conversion of that species to a product that can be isolated completely and weighed (Gammon et. al, 2009). Most traditional gravimetric methods require the knowledge of stoichiometric reactions, solubility rules and the calculation of mass of substance. Gravimetry  is comprised of sub-procedures such as precipitating the sample, filtering the solution, washing the precipitate free of contaminants, igniting the precipitate and finally weighing the precipitate and determining its mass by difference. Precipitation is a process in which the sample is reacted with another sample to form an insoluble product which is called the  precipitate while the manner of separating the precipitate from the mother liquor is filtration. It is necessary to assure that the precipitate is free from impurities within, large enough to filter and negligibly soluble. Washing of precipitate with liquid removes all soluble impurities sticking with the precipitates (Hage et. al, 2011). After separation, the substance must undergo ignition  before weighing by heating up the precipitate to drive off excess solvent and volatile electrolytes  but it is subjected to change the chemical composition of the precipitate. An advantage of gravimetric analysis is that identifying the mass of a substance is one of the most accurate measurements that can be made with errors of less than 0.2% (Hage et. al, 2011). This method of analysis has a real life applications such as the determination of chemicals in contaminated water, amount of fat a food may contain, chemical analysis of ores and other industrial materials, in the calibration of instruments, and in the elemental analysis of inorganic compounds and measurement of the essential elements in plant foods. Although the process is time-consuming and tedious, the method guarantees an accurate result. In this experiment, the Gravimetric Determination of Iron , the purpose is to define the  principles and standard techniques involved in precipitation and gravimetric analysis. It aims to  2 obtain the percent composition of the analyte, which is Iron, in an unknown sample using gravimetric data. Gravimetric factor is defined to be the algebraic expression that converts grams of a compound into grams of a single element. It is the ratio of the formula weight of the substance being sought to that of the substance weighed. The formula for gravimetric factor  is:          ,where m and n are molar masses  .   On the other hand, the formula for the percentage of an element in a sample is given as:            where,          ()   ()           The experiment is designed to measure whether the techniques in precipitation and gravimetric analysis were properly performed by the analysts in order to yield an accurate outcome. Thus, in this experiment, one will analyze the amount of an iron in a given sample by  precipitating, from basic solution, the hydrated Iron Oxide. The reaction is immediately followed  by a dehydrated reaction to produce the solid Fe 2 O 3. The gelatinous hydrous oxide can block impurities. Therefore, the initial precipitate is dissolved in acid and re-precipitated. Because the concentration of impurities is lower during the second precipitation, occlusion is diminished (Harris, 2003).  3 II. Methodology Constant Mass of the Crucible In the beginning of the process, two crucibles were labeled as crucible A  and crucible B . Both crucibles were weighed so that the initial mass can be bases of comparison to the mass of the crucibles when heated. The crucible A  should be heated in a way of bringing the flame in and out every two seconds. When the crucible glowed orange, the heating continued for 10 more minutes while the ceramic supports on the clay triangle also glowed. After heating, the set-up was cooled until the crucible was at room temperature. The crucible was placed in a dessicator which was then brought to the balance room. The crucible was taken out of the dessicator and was weighed in an analytical balance. After recording the mass, the crucible was placed back in the dessicator to keep the crucible dry. If the crucible were not  placed in the dessicator it is subjected to moisture and, therefore, can make the mass of the crucible heavier since what was weighed was the mass of the crucible plus moisture. The steps done with crucible A  were repeated with crucible B . Alternately, while crucible B  was being cooled down, the 2 nd  trial for crucible A  was being performed. The process was continued until the 2 nd  trial for crucible B  was performed. Successive weighing must agree within 0.3 mg. Gravimetric Determination of Iron Two trials were performed in this part of experiment. In the first trial, the obtained mass of the sample, named  sample A , was 0.546g and then placed in a 400-mL beaker. For the second trial,  sample B  with a mass of 0.606g was also placed in a 400-mL beaker. Both samples were added with 15-mL water and 10-mL of 3M HCl. Both samples were not filtered and, therefore, was added with 5-mL of 6M HNO 3  to the solution and boiled for a few minutes until the solution becomes clear yellow. Both samples were diluted to 200-mL distilled water and were added with 3M of NH 3 . The solutions were constantly stirred until it was basic. The  basicity was determined through the use of litmus paper. The precipitate was then digested by  boiling for 5 minutes. Since both samples did not boil, the solutions were heated vigorously for 25 minutes and then allowed the precipitate to settle. In filtering both solutions, an ashless filter paper was used to each in order to avoid contamination of the sample. Note that  sample A  and  sample B  were not filtered simultaneously due to unavailability of equipments. The ashless filter paper was wetted to  4 make it stick to the funnel. The supernatant liquid of both solutions were decanted not higher than 1 cm from the top of the funnel. All solid from the beaker were quantitatively transferred to the filter paper through the use of rubber policeman and hot 1% (w/w) NH 4  NO 3 . The filter  paper that contained the precipitate was drained thoroughly until it was ready to lift out of the funnel. The filter paper was folded in a manner in which it must be flatten, the edges were folded then the top and finally placed inside the crucible with point pushed against bottom. The crucibles were properly labeled with  sample A  and  sample B . The crucibles that each contained the samples were left in a container for two nights. The experiment was then continued, starting with the process of ignition. Each uncovered crucible was subjected to heating with full heat coming out from the burner to completely burn the filter paper. Through the use of crucible tongs, the cover was placed on the crucible, which contained the precipitate formed in  sample A,  every time the filter paper inside the crucible was caught with flame. However, the cover was also removed every after a few seconds to have access to air to avoid turning the carbon into graphite or to avoid reduction of iron. Moreover, the crucible was also moved every once in a while so that the heat was not concentrated on one side of the base. By the time, the filter paper was charred, the position of the crucible was moved in a manner that the flame evenly touches the surface of its base. It was subjected to full heat for about 15 minutes to ensure complete ignition of the Iron Oxide. When ignition was completed, the crucible was cooled in air until it reached room temperature level  before it was transferred to the dessicator. The crucible and the lid were individually weighed. The sum of the recorded mass of crucible and lid was then subtracted to the constant mass of the heated crucible A without the precipitate and the difference was the mass of Iron Oxide. The process was repeated for the second trial of crucible A, first trial of   crucible B and second trial of   crucible B with both trials in crucible B containing the precipitate were each subtracted to the constant mass of the heated crucible B  without the precipitate . After the weight percent of iron in each sample was obtained, the average and average deviation were then calculated.
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