Gravimetric analysis
Introduction
Gravimetric analysis is a method of measuring of mass of chemical substances. This analysis can be classified in two types; precipitation and volatilization (Bassett et al.). In precipitation method some steps, involved to determine quantify of the chemical substances, are separation of an ion in aqueous solution by a reaction, filtering and washing the precipitate, converting the precipitate to a substance of known composition, and lastlytaking weight of the precipitate as well asevaluating its mass. The chemical substance must be precipitated fully.Theunknown amount of original ion can be evaluated from the mass and known composition of the precipitate. Gravimetric method enjoys some advantages like it is accurate, less complex etc. The objective of this lab experiment of “Gravimetric analysis” (Brady et al. 2011) was to demonstrate typical methods used in gravimetric analysis by quantitatively evaluating the amount of chloride in an unknown.
Procedure:
The procedure (Brady et al.) of experiment has been described in following paragraph.
At first an unknown was obtained and its number was recorded on report. On a piece of weighting paper, the mass to the nearest 0.0001 g about 0.2 to 0.4 g of unknown sample was determined. The sample was then quantitatively transferred to a clean 250 mL beaker and a pencil was used to label the beaker #1. The sample mass was recorded. After this, 150 mL of distilled water 1 mL of 6 M HNO3was added to the beaker. Above procedure was repeated with samples 2 and 3, labeling the beaker #2 and #3, respectively. Using a different glass rod for each solution, contents of the beaker was stirred until the entire sample was dissolved. While stirring one of the solutions, 20 mL of 0.5 M AgNO3 solution was added. A watch glass was placed over the beaker. The solution was then gently warmed with Bunsen burner and it was kept warm for 5-10 minutes. After this, a filter paper for each beaker was taken and mass of it was determined after it had been folded and torn. Filter paper was fitted in glass funnel and it was opened in the funnel so that one side has three pieces of paper and one side has one piece of paper against the funnel – not two pieces on each side. In the next step, the paper was made wet with distilled water to hold it in place in the funnel. The precipitate and all of the warm solution were completely and quantitatively transferred from the beaker onto the filter using a rubber policeman and a wash bottle to remove the last traces of precipitate. Then the precipitate on the filter paper was washed with two or three 5 mL portions of water from the wash bottle. At the end, three 5 mL portions of acetone were poured through the filter.
Above steps were repeated for other two samples. The precipitated AgCl was kept out of bright light because it is photosensitive and slowly decomposes in presence of light as follows:
2AgCl(s) → 2Ag (s) + Cl2 (g)
When the AgCl was completely dry, mass of filter papers and AgCl were determined and mass of the AgCl was calculated. Using these data, percentage of chloride was determined.
Result:
Following table indicates the mass of sample, filter paper and AgCL which were obtained during the lab experiment.
g Cl = (0.2473 g Cl/g AgCl ) x ( 0.591 g AgCl) ( where the number 0.2473 is gravimetric factor)
= 0.146 g Cl
So, % Chloride in original sample = ( 0.146 g Cl) / (0.234 g sample) x 100 % = 62.46 %
Like the same way, mass of Cl in original sample (g) for Trial -2 =
g Cl = (0.2473 g Cl/g AgCl ) x ( 1.092 g AgCl) = 0.27 g Cl
So, % Chloride in original sample = ( 0.27 g Cl) / (0.441 g sample) x 100 % = 61.24 %
Average % chloride = (62.46 % + 61.24 %)/2 = 61.85 %
Deviation from mean for Trail -1 = | 62.46 – 61.85 | = 0.61
Deviation from mean for Trail -2 = | 61.24– 61.85 | = 0.61
Standard deviation = [( 2 x 0.61^2 )/(2-1) ]^0.5 = 0.8626
Average deviation from the mean is = 0.61
So, relative standard deviation expressed as percentage is = 0.61/61.85 = 0.986 %
Discussion
Probable sources of error were low mass due to dropping the substance; low mass due to decomposition in presence of light; reaction between water and silver nitrate as tap water was used accidently etc.
The experiment was carried out safely. Two trials of the experiment were carried out and in both cases almost similar results were found. Average percentage Chloride in original sample was measured 61.85% by performing this experiment. Standard deviation and relative standard deviation were considerably low which indicated that the lab experiment was carried out successfully and amount of error was negligible. The result, obtained from the lab experiment was satisfactory and less affected by errors. By carrying out this experiment knowledge about gravimetric analysis was gained and here lies the success of the experiment.
Reference
Brady,J.E, Jespersen, N.D.,Hyslop,A.2011.Chemistry The Molecular Nature of Matter, 6th Ed
Brown, T. E., Eugene, H., LeMay, H., Bursten, B.E. Catherine. Laboratory Experiments Chemistry the central science, 13th Ed
Bassett, J.; Denney, R. C.; Jeffery, G. H. Mendham. J. Vogel’s Textbook of Quantitative Inorganic Analysis. Longman, London.408
