LAB 2: ANALYSIS OF A STERLING SILVER BRACELET
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This lab requires a lab report.
You will work with a partner for this lab.
I Introduction
Spend any amount of time at a jewelry counter and you’ll soon find that pure silver is
relatively uncommon in jewelry. Why? Like gold, pure silver is too soft to form
relatively large, durable items like jewelry, and so must be alloyed with another
metal to provide strength. Therefore, instead of pure silver necklaces and bracelets,
you’ll likely find pieces made of sterling silver, an alloy of silver and copper. In this
lab, we will determine the percent silver in a sterling silver necklace.
To determine the mass percent of silver in the necklace, we will do the following (in a
nutshell):
1) We will chop the necklace into tiny pieces and dissolve it in concentrated
nitric acid. This will oxidize the silver metal (Ag(s), oxidation state = 0) to
the silver ion (Ag+(aq), oxidation state = +1). Of course, it will also oxidize
the copper metal in the alloy to the copper(II) ion, Cu 2+.
2) We will separate the dissolved silver and chloride ions, however, by adding
sodium chloride, which will precipitate the silver ions as solid AgCl but will
not precipitate the copper ions (CuCl2 is soluble).
3) We will then filter out the silver chloride precipitate from the solution and
find its mass. All of the silver that was in the bracelet is now captured in
this precipitate.
4) From the mass of silver chloride formed we will calculate the mass of silver
present in the precipitate and thus (originally) in the necklace.
5) From the mass of silver calculated and the mass of the original sample, we
will then calculate the percent of silver in the alloy.
Because these results are based on the mass of a product (which we find by weighing,
which, of course, involves gravity), this procedure is an example of a gravimetric
analysis. Gravimetric analysis is a standard “classical” (i.e. low-tech) method for
determining the amount of a given component present in an unknown sample. The
method involves precipitating the component of interest (in this lab, silver) from the
unknown sample (e.g. sterling silver bracelet) by means of some added reagent (such
as sodium chloride). From the mass of the precipitate, the percentage of the unknown
component in the original sample is calculated.
Silver and copper are very non-reactive metals. Neither will readily dissolve in
hydrochloric acid or sulfuric acid. Instead, the “oxidizing” acid, nitric acid (HNO 3) is
required. In acidic solutions, the nitrate ion (NO3-) is an excellent oxidizer, and it will
oxidize Ag(s) to Ag+(aq) and Cu(s) to Cu2+(aq). The reduction product is nitrogen
monoxide (NO), a colorless gas. This is summarized in the following unbalanced
equation:
Ag(s) + HNO3(aq)  AgNO3(aq) + NO(g) + H2O(l)
As the colorless nitrogen monoxide gas forms, however, it immediately reacts with
oxygen in the air to produce the orange-brown gas nitrogen dioxide (NO2):
2 NO(clear gas) + O2(clear gas) →2 NO2(brown gas)
This brown gas will be quite visible to you. It’s also quite toxic to you, so be sure to
perform this part of the experiment in the hood.
Equipment and Reagents
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Beakers, 100-mL and 250-mL
Erlenmeyer flask
Graduated cylinder
Watch glass
Stirring rod and wash bottle
Ringstand, ring, and funnel
Analytical balance
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Sterling silver necklace
6 M HNO3 solution
0.20 M NaCl solution
Distilled water
! Warnings!
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The nitric acid we are using (6 M) is extremely corrosive. Wear goggles and
aprons when using and use only in a fume hood. Neutralize any spills
immediately with Acid Lock®.
Solutions containing silver ions cause dark stains on skin or clothes which do
not appear for several hours. Wear your aprons and clean up any spills
immediately with plenty of soap and water.
Wear your goggles and apron, for the above reasons cited.
The gaseous products, NO and NO2, are both toxic. Do not inhale these gases
and be sure they are adequately vented to the birdies outside.
This is a quantitative lab that requires ALL of the silver initially present in the
alloy to end up precipitated in the form of silver chloride. It’s therefore
essential for you to completely transfer all substances when needed, down to
the last drop of solution and last speck of precipitate.
Procedure
1. Set up your lab notebook.
2. Obtain a sample of the sterling silver bracelet that is between 0.1 and 0.5 g.
Determine its mass precisely on the analytical balance and record it.
3. Place the bracelet sample in a clean 100-mL beaker. IN THE HOOD, carefully
pour 10 mL of 6 M nitric acid over it. Cover the beaker with a watch glass so
none of the solution spatters out. It may be necessary to gently heat the
solution so that the alloy dissolves. Allow the alloy to totally dissolve. Record
your observations.
4. Measure out 50 mL of sodium chloride solution. It does not have to be exactly
50 mL; we’re adding a two-fold excess of it to ensure “complete”
precipitation.
5. Slowly add the sodium chloride solution to the dissolved silver solution. Stir
with a stirring rod and rinse any solution clinging to the rod back into the
beaker. Record your observations and set the beaker aside a few minutes to
allow the particles to grow larger and thus more “filterable”.
6. Setup a filtration apparatus (ring stand, ring, funnel, Erlenmeyer flask, etc.).
Prepare a piece of fluted filter paper AND RECORD ITS MASS.
7. Prepare a rinse solution in a beaker by dissolving 2 mL of 6 M HNO3 in 150 mL of
distilled water. Label your beaker appropriately and set aside for later use. The
acid in the rinse water helps the precipitate from “peptizing”, or forming
extremely small particles that will run through the filter.
8. Returning to your solution containing your dissolved necklace sample, carefully
pour the solution down a stirring rod into the funnel. Then wash the precipitate
into the funnel with your rinse solution and a rubber policeman. Be sure to get
every particle! Rinse the precipitate several times with the rinse solution.
9. Set the filter and precipitate on a paper towel and place in the designated
cabinet to dry overnight. Silver ions (including those in AgCl) are
photosensitive: when exposed to light, they are reduced to silver metal when a
reducing agent is present.
10. Since your filtrate contains only ions of sodium, copper, and nitrate, you can
safely pour it down the drain, flushing it with plenty of water.
Procedure, Day 2
11. Weigh and record the mass of your precipitate. Determine the percent silver in
your sterling silver necklace.
12. Dispose of your silver chloride product in the designated waste beaker.
Data and Calculations (to be recorded on your lab Data Sheet)
Day 1
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Title and Purpose
Procedure used
Mass of necklace sample
Observations after addition of HNO3
Observations after addition of NaCl
Mass of filter paper
Day 2
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Procedure used
Mass of AgCl precipitate + filter paper
Mass of AgCl precipitate
Mass of Ag in sample
% Ag in sterling silver necklace
? Questions to Answer
1) Draw a flowchart tracing the “path” of the silver throughout the experiment,
beginning with the silver in the alloy and ending with the silver in the
precipitate. Include reagents and procedures used.
2) Draw a flowchart showing how to calculate the percent silver in the alloy,
beginning with the mass of AgCl and ending with the %Ag in the alloy.
3) Why is it possible to analyze the silver content (and only the silver content) of
a silver-copper alloy by precipitating with chloride ion?
4) If the silver chloride is not dry when its mass is determined, will the calculated
percent of silver in the alloy be too high, too low, or unchanged? Explain.
5) Will the nitric acid in the rinse solution affect the mass of the silver chloride
precipitate as it dries? (Nitric acid is prepared by dissolving the gas N 2O5 in
water.)
6) Why don’t we just use hydrochloric acid to both dissolve and precipitate the
silver?
7) The standard composition of sterling silver is 92.5% silver and 7.5% copper.
Compare this to your results and calculate your percent error.
8) A silver-copper alloy had a mass of 0.1264 g. When the alloy was dissolved in
nitric acid and the silver precipitated as silver chloride, the precipitate had a
mass of 0.1375 g. Calculate the percent of silver in the alloy. Show your
calculations.