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Dec 6, 2023

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Gravimetric Analysis of a Chloride Salt Short Report Date submitted: By: Lab section: Group: TA:
Procedure: Weighing the salt samples by difference: 1. The vial of salt sample 337 was placed into the analytical balance and the balance was zeroed to measure the mass of salt content removed. 2. A small amount of salt was removed from the vial and placed into a 250.00 mL beaker and the vial was placed back into the balance. 3. The mass salt removed from the vial was deduced from the zeroed value on the analytical balance. 4. The process was repeated until a sample value of about 0.1000 g to 0.1500 g was collected into each of the two 250.00 mL beakers. a. In this particular experiment, 0.1306 g ± 0.0001 g of salt sample 337 was measured and collected into beaker 1 and 0.1015 g ± 0.0001 g was collected into beaker 2. Preparation of solution: 1. Both 250.00 mL beakers were filled to approximately 100.00 mL with distilled water and the solutions were stirred with a glass rod. 2. 1.00 mL of HNO 3 was added into each solution and stirred. 3. Using the calculated masses, the volume of AgNO 3 required for each salt sample was calculated and added to its respective solution from a 100.00 mL beaker using a 50.00 mL graduated cylinder. 4. The solutions were mixed with a glass stir rod. Precipitation test and storage: 1. Each salt sample was placed on a hot plate which was turned onto medium heat, making sure the solution did not boil. 2. Each sample was stirred occasionally, without removing the glass stir rod. 3. A few drops of AgNO 3 were dropped into each salt sample to test for completion. a. If too cloudy or precipitate formed, then supernatant remained on hot plate until no precipitate formed. 4. Once passed the precipitation test, the beakers were removed from the hot plate and placed into a drawer until cooled. Filtration and washing of the precipitates: 1. Without folding, a piece of filter paper was placed into each of the two 50.00 mL beakers and then massed using the analytical balance. 2. The filter paper from beaker one was inserted into one of the Buchner funnels of a vacuum filtration apparatus and soaked with distilled water. a. The same was done for filter paper number 2 in the second funnel. 3. Once the water had passed through, the supernatant liquid for beakers 1 and 2 were decanted through their respective funnels. 4. To wash the precipitates, several mL of 0.01M HNO 3 was decanted through the precipitates and the filters a total of three times.
5. After stopping the vacuum, the waste filtrates were thrown out and the apparatus was then reassembled and turned on. 6. Another several mL of HNO 3 were poured over the precipitates in the Buchner funnels. Once the acid had passed through, the vacuum was stopped and unassembled again. 7. Several mL of the washing was poured into two separate test tubes, making sure to not fill them completely, and 3 drops of HCl were pipetted into the tubes to test for completion of wash test. a. If any cloudiness was visible in the tubes, steps 1-7 in the washing procedure were repeated until the washings remained clear. 8. The contents of the test tubes were discarded into the sink and the tubes were cleansed with distilled water. Excess wash from the filter glasses were also discarded in the sink. 9. Once the vacuum was reassembled and turned on, the precipitates were covered with acetone. Once the liquid had passed through, this process was repeated 2 more times. 10. Once the precipitates were dry, the vacuum was turned off and the apparatus was disassembled. The acetone glasses were then disposed of in their respective container. Drying and massing of precipitates: 1. Over a piece of paper towel, the filter papers were carefully removed from their funnels using a micro spatula and transferred into their respective 50.00 mL beakers, making sure all solids went into the beaker. 2. Each 50.00 mL beaker was placed into an oven at 86.0 °C for 13 minutes. 3. After the samples had cooled for 4 minutes, each beaker with its precipitate and filter paper was massed using the analytical balance. 4. Once values were measured, the filter papers and precipitates were disposed of and all instruments were thoroughly cleaned using distilled water. Data and Observations: 1. Once the AgNO 3 was added to the solutions, they became a cloudy, opaque white. a. Grey precipitate formed in the solutions instantly. 2. If the samples failed the precipitation test for completion, grey, clumpy matter would form in the solution. 3. Several grams of precipitate filtered through the vacuum while the washing test was being conducted. 4. A small amount of precipitate was left stuck to the 250.00 mL beakers and the Buchner funnels, unable to be rinsed or scraped off into the final massed samples. 5. The temperature of the oven quickly rose from 80.0 °C to 86.0 °C within 3 minutes of placing the 50.00 mL beakers inside. The temperature remained relatively consistent throughout the rest of the drying process. Table 1: Initial and final description of the physical changes of each sample Sample Initial Physical Description (salt) Final Physical Description (precipitate)
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Number 1 - White, grain-like powder - White, grain-like powder 2 - Clumped light purple powder - Clumped light purple powder Table 2: Representing the check for completeness of each test per sample Sample Number Completeness of Precipitation Test Completeness of Washing Test 1 - No additional AgCl formed - Translucent solution - Cloudy, white precipitate forms on first wash - No turbidity observed after second wash 2 - Precipitate formed quickly in clouded solution - No additional AgCl formed on the second trial - No turbidity observed Table 3: Data of the weight of the samples and precipitates Sample Number Mass of Salt Sample 337 (±0.0002 g) Mass of the Beaker with Filter Paper (BF) (±0.0002 g) Mass of the Beaker with Filter Paper and the Precipitate (BFP) (±0.0002 g) Mass of the Precipitate (±0.0004 g) 1 0.1306 29.79 29.96 0.1716 2 0.1015 28.90 29.16 0.2596 Table 4: Data of the oven temperature (in degrees Celsius), drying time (in minutes), and cooling time (in minutes) Sample Number Oven Temperature ( °C) Drying Time (mins) Cooling Time (mins)
1 86.0 13.0 4.00 2 86.0 13.0 4.00 Table 5: Instrumental uncertainties of measured and transferred additives Sample Amount Transferred/Measured Instrumental Uncertainty 1 0.1306 g salt 337 100.00 mL distilled water 1.00 mL HNO 3 25.2 mL AgNO 3 29.7864 g BF 86.0 °C 29.9580 g BFP ± 0.0001 g (analytical balance) ± 0.12 (250.00 mL beaker) ± 0.01 (5.00 mL pipette) ± 0.2 (50.00 mL graduated cylinder) ± 0.0001 g (analytical balance) ± 0.2 (oven thermometer) ± 0.0001 g (analytical balance) 2 0.1015 g salt 337 100.00 mL distilled water 1.00 mL HNO 3 20.7 mL AgNO 3 28.9014 g BF 86.0 °C 29.1610 g BFP ± 0.0001 g (analytical balance) ± 0.12 (250.00 mL beaker) ± 0.01 (5.00 mL pipette) ± 0.2 (50.00 mL graduated cylinder) ± 0.0001 g (analytical balance) ± 0.2 (oven thermometer) ± 0.0001 g (analytical balance) Calculations: Use the following equations to calculate the volume of AgNO s (aq) + 5.00 mL required for the precipitation of salt sample 337 as well as the percent uncertainty: 0.1M AgNO 3 ¿ ( ( ( Sample Mass × % Cl¯ Atomic massof CL ) 0.1 M ) × 1000 L ) + 5.00 mL % uncertainty = ( Instrumental Uncertainty Sample Mass ) × 100
Sample calculation using data for sample number 1: 0.1M AgNO 3 ¿ ( ( ( 0.1306 55% 35.5 g mol ) 0.1 M ) × 1000 L ) + 5.00 mL = 25.2338 mL 25.23 mL Approximately 25.23 mL AgNO 3 is required for sample 1 and 20.73 mL is required for sample 2. %uncertainty = ( Salt MassU ncertainty Sample Mass 1 ) × 100 = ( 0.0002 g 0.1306 g ) × 100 ± 0.15 % The percent uncertainty of the volume of AgNO 3 required is approximately ± 0.15 % for sample 1 and ± 0.20 % for sample 2. Calculation of the percent chloride in each sample: The moles of chloride in each sample were calculated using the following equation: Using values gathered from sample 1: Moles Cl 1 ¯ = ( Mass of Precipitate 1 Molar Mass of AgCl ) = ( 0.1716 g 143.32 g mol ) 0.001197 mol There are 0.001197 mol Cl - in sample 1 and 0.001811 mol Cl - in sample 2. %uncertainty = ( Precipitate MassUncertainty Precipitate Mass 1 ) × 100 = ( 0.0002 g 0.1716 g ) × 100 ± 0.23 % In moles: uncertainty = ( % Uncertainty 100% ) × Moles Cl 1 ¯ = ( 0.23% 100% ) × 0.001197 mol ± 0.000003 mol The percent uncertainty of the volume of AgNO 3 required is approximately ± 0.23% or ± 0.000003 mol for sample 1 and ± 0.15 % or ± 0.000003 mol.
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The mass of chloride was derived as shown below: Using values gathered from sample 1: Mass Cl 1 ¯ = MolesCl 1 ¯ × Molar Mass of Cl = 0.001197 mol × 35.5 g mol 0.0425 g The mass of chloride in sample 1 is 0.0425 g and in sample 2 it is 0.0643 g. % uncertainty = ( Molaruncertainty MolesCl 1 ¯ ) × 100 = ( 0.000003 mol 0.001197 mol ) × 100 ±0.23% In grams: uncertainty = ( % uncertainty 100% ) × Mass Cl 1 ¯ = ( 0.23% 100% ) × 0.04250 g = 0.000099 g uncertainty ± 0.0001 g The percent uncertainty of the mass of chloride in sample 1 is approximately ± 0.23% or 0.0001 g and ± 0.15% or 0.0001 g for sample 2. Using the following equation, calculate the percent chloride in each sample: Sample calculation using values gathered from sample 1: % Cl 1 ¯ = ( MassCl 1 ¯ Sample Mass 1 ) × 100 = ( 0.0425 g 0.1306 g ) × 100 32.54 % The percent chloride in sample 1 is about 32.54 % and in sample 2 it is 63.35%. %uncertainty = (% uncertainty of Mass Cl 1 ¯ ) + ¿ (% uncertainty of Sample Mass 1 ) = ( 0.0001 g 0.0425 g ) × 100 + ( 0.0002 g 0.1306 g ) × 100 = 0.23 % + 0.15 % ± 0.39 % The uncertainty of the percent chloride in each sample solution is approximately ± 0.39% for sample 1 and ± 0.35% for sample 2.
The average percent of chloride in both samples is calculated with the equation below: % Cl ¯ ave = ( % Cl 1 ¯ + % Cl 2 ¯ ) 2 = ( 32.54% + 63.35% ) 2 47.95 % %uncertainty = ( % Uncertainty 1 + % Uncertainty 2 ) 2 = ( 0.39% + 0.35% ) 2 ± 0.37% The average percent of chloride in both samples is approximately 47.95 ± 0.37 %. Calculation of the relative error was done using the following equation: relative error ¿ experimental value accepted value accepted value × 100% relative error ¿ 47.95% 54.11% 54.11% × 100% 11.39% relative error ≈ 11.39 % Relative spread of the chloride percentages was calculated using the following: relative spread ¿ high value low value ¿ average value × 1000 ppt ¿¿ relative spread ¿ 63.35% 32.54% ¿ 47.95% ¿¿ × 1000 ppt ≈ 642.45 ppt r elative spread≈ 642.5 ppt Discussion: This experiment provided a final value of 32.54 ± 0.39 % chloride percent for sample one and 63.35 ± 0.35 % for sample 2. Using this data, a final average chloride percentage of approximately 47.95 ± 0.37% was calculated, which was about 11.39% lower than the expected result of 54.11%. One reason for this could be due to the photocomposition of the AgCl, which can be seen through the faint purple colouring of the precipitate, as caused by incomplete precipitation from escaped chlorine gas. Another possibility would be undue agitation leading to additional escaped chlorine gas. The surface of a dried precipitate is protected by solid silver but agitating
it could terminate this effect of the silver “coat” during photodecomposition, creating more chlorine gas. Leading to a third cause, while heating the solutions to trigger the formation of crystalline structures, the precipitate test could have been incomplete, which would have triggered additional substance loss through the filter as they would not have been big enough to be held back. This was observed in the filtration procedure. As well, during precipitate transfer from the 250.00 mL beaker and the Buchner funnel, small but noticeable amounts of precipitate were left behind, unable to be removed or scraped into the target instrument. This would have also contributed to the lower chloride percentage. Conclusion: To conclude, the percent of chloride in the samples collected of salt 377 were determined to be approximately 32.54 ± 0.39 % for sample 1 and 63.35 ± 0.35 % for sample 2, with a final average percentage of approximately 47.95 ± 0.37%. The results presented a relative error of about 11.39 % lower than the given value of 54.11 %, which could have been attributed mainly to sources of error in the precipitation and/or washing tests leading to the escape of chlorine gas. Additionally, the relative spread of the chloride percentages was calculated to be about 642.45 ppt, exhibiting an insufficiency in accuracy with the data collected.
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