Components of mixtures or compounds can often be analyzed by reacting them with something else to produce a compound of one of the components, and then determining the quantity of that compound produced. This method can be used to determine the amount of the original component in the mixture or compound.
In this lab, you will be analyzing the silver content of a US dime minted prior to 1965.
Dissolving the alloyWeigh your silver alloy sample on the analytical balance. Then place the sample in a 100 mL beaker (labeled with your name or initials and period), and place it under the fume hood. Carefully add 20 mL of 6 M HNO3 to the beaker, and place it on a hotplate. Put a watch glass on top of the beaker and warm gently. Allow the sample to fully dissolve, and record any observations of the process. When fully dissolved, remove from the hotplate and allow it to cool for 5-10 minutes. Leave under the fume hood as it is cooling.
Precipitating the silver as silver chlorideDetermine the mass of NaCl needed to react with the silver ions in solution (assume that the sample is 100% silver), and double that amount. Weigh that much NaCl on a regular electronic balance, and place it in another 100 mL beaker. Add distilled water to the 20 mL mark, and stir to dissolve the salt. When the alloy solution is cool, add the NaCl solution to it and gently stir it with a stirring rod. When finished stirring, rinse the rod with distilled water over the beaker to make sure none of the AgCl escapes the beaker. Cover the top of the beaker with Parafilm and allow it to sit undisturbed overnight - this allows the particles to aggregate, creating larger particles. (Wrap the beaker with aluminum foil to protect the AgCl from the light.)Recovering the silver chlorideFilter the AgCl using gravity filtration on a pre-massed piece of filter paper. Be sure to put your initials and period number (in pencil) on the underside of the filter paper before you mass it. Rinse the AgCl and the filter paper several times with distilled water to which a small amount of nitric acid has been added. When all of the precipitate is in the filter paper, rinse the stirring rod and the beaker until no traces of precipitate remain on them. Allow all of the liquid to go through the filter paper, then rinse the filter paper and precipitate. When all of that liquid has gone through the filter paper, rinse the paper and precipitate again. Perform this rinse procedure a total of 3-4 times. Once the last rinse has gone through the filter paper, remove it from the funnel and place it on a watch glass and unfold it. Place it in a closed cabinet (so it will be in the dark) and allow it to dry over the weekend, then weigh the paper and precipitate.
Results and Discussion:
You need to calculate:
Discuss what experimental factors could have caused errors in determining the percent silver in the dime you had. Pay attention to the sign of the error - in other words, what could have caused you to get a larger or smaller percent Ag content than the accepted value. What other evidence did you have that another metal was a part of the dime's composition? Give a balanced formula equation for the reaction between silver nitrate and sodium chloride (include physical states), and show the net ionic equation for the same reaction. http://www.treasurerealm.com/coinpapers/USCoins/dime.html http://www.answers.com/topic/dime
- the amount of NaCl needed to react completely with the silver in your sample (assuming your sample is 100% silver)
- the amount of AgCl produced, the amount of Ag in the AgCl, and the percent of Ag in the original sample (by mass)
- the absolute and relative errors for your experimental value of the percent Ag versus the accepted value for the percent Ag in a pre-1965 US Roosevelt dime (see websites below for the accepted value)
Send questions, comments or suggestions to
Gwen Sibert, at the
Roanoke Valley Governor's School
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