The purpose of this lab was to calculate the percent of silver in an alloy using gravimetric analysis. Through the procedure mentioned in this report, the percent of silver in an alloy of a U.S. Mint dime made before 1965 was 90.2% � 0.05%.
The alloy used for this experiment was a U.S. Mint ten-cent dime alloyed as silver and copper if made before the year 1965, after which mints began using copper and nickel instead. First, the dime piece was dissolved in nitric acid and the silver was precipitated as silver chloride.
The precipitate was then filtered, washed, dried and weighed. To obtain the percent of silver in the alloy, the mass of silver chloride was compared to the mass of the original sample. Because the results are based on the mass of a product, the procedure is determined as gravimetric analysis.
Nitric acid (HNO3) is required as an “oxidizing agent” because silver and copper are very non-reactive metals and cannot dissolve in hydrochloric acid or sulfuric acid.
The nitrate ion (NO3�) oxidized Ag(s) to Ag+(aq) and Cu(s) to Cu2+ (aq). As the nitric acid was reduced, the product became nitrogen monoxide (NO), a colorless gas that instantly reacts with oxygen in the air to produce nitrogen dioxide (NO2), an orange-brown gas. The following demonstrate the half reactions mentioned:
To separate the silver ions and the copper ions, the silver was precipitated as silver chloride (AgCl) by adding dissolved sodium chloride (NaCl) to the solution, which also created the product of copper (II) chloride (CuCl2).
Theoretically, because copper (II) chloride is soluble in water and silver chloride is insoluble in water, the addition of chloride ions to the solution from the sodium chloride precipitated all the silver ions and none of the copper ions. The silver chloride precipitate was filtered from this solution. The following demonstrates the reaction for the precipitation mentioned:
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Amount of NaCl necessary to precipitate the silver x2
Mass of Ag (dime*) x 1 mol Ag x 1 mol NaCl x 58.443g NaCl
107.868g Ag 1 mol Ag 1 mol NaCl
Assuming that the dime is 100% silver
0.309g Ag x 1 mol Ag x 1 mol NaCl x 58.443g NaCl = (0.167 � 0.001)g NaCl
107.868g Ag 1 mol Ag 1 mol NaCl
2(0.167) = (0.334 � 0.002)g NaCl*
*0.332 � 0.001g NaCl was used in the experiment
Mass of AgCl
(Mass of Filter + AgCl) – Mass of Filter
0.435g – 0.067g = (0.368 � 0.002)g AgCl
Mass of Ag
Mass of AgCl x Molar Mass Ag = %Uncertainty = Uncertainty x 100
Molar Mass AgCl Value
1.368g AgCl x 107.87g Ag =(0.277 � 0.001)gAg (0.5%)%Uncertainty = 0.002 x 100 = 0.5%
143.32g AgCl 0.368
Percent Silver in Dime
Mass Ag x 100 =
Mass Dime
0.277g Ag x 100 = (89.6 � 0.7)% 0.309g Dime
%UncertaintyDime =Uncertainty = 0.001 x 100 = 0.3%
Value 0.309
%Uncertainty%Ag in Dime = %UncertaintymassAg + %UncertaintyDime =0.5% + 0.3% = 0.8%
Therefore, 0.8% of 89.6% = 0.008 x 89.6 = 0.7%
By means of the experiment procedure, the mass of the dried precipitate (AgCl) and filter paper was obtained. Next, the mass of AgCl precipitate and eventually the mass of silver itself was calculated. Through dividing the acquired mass of silver by the original mass of the dime, the percent of silver within the dime was achieved.
However, not all of the silver from the original sample was and could be accounted for. Because the funnel from Day 2’s procedure was not covered in plastic wrap before being left in the drying oven overnight, dust particles possibly could have collected in the filter in addition to the precipitate. This would offset the measurement of the mass of precipitate on Day 3. Furthermore, during Day 2’s procedure, there was the risk that the precipitate had “peptized”, forming minute particles that could pass through the filter due to the use of the wash bottle to wash the precipitate into the funnel with the dilute nitric acid solution.
This problem is solved if measures such as 2 mL of 6M HNO3 had been added to the 150mL of distilled water in the wash bottle. The outcome would not have shifted greatly, but it would have avoided the possibility allowing all of the particles to pass through. Additionally, it is my opinion that if the lab spanned a time slot such as 2 hours to complete the lab and the written report, the quality of the results would have more reputability because it was all done at once.
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