Name
Lab Day
Analysis of a Solution Containing Silver Ions
Introduction:
Metallic copper and aqueous silver ion react to form aqueous
copper ion and elemental silver. This process may be viewed as a transfer
of electrons. A copper atom donates electrons to two silver ions. This can
be summarized as:
Cu0 + 2 Ag+ → 2 Ag0 + Cu+2
For every mole of copper converted to copper ion, two moles of
silver ion are converted to elemental silver. By knowing the volume of a
solution containing silver ion and determining the weight of copper
reacted or the amount of silver formed, the amount of silver in the
solution may be calculated.
Experimental:
Obtain a piece of copper wire about 20 cm long. Clean the wire by
polishing it with steel wool. Loosely coil the wire about a pencil and then
weigh it to at least 0.01 g. Record your data and observations on the data
page.
Select one of the unknown solutions containing silver. Record the
unknown’s number or code in the data section. Pour 50 mL (to the
nearest 0.5 mL) of the silver solution into a clean small beaker using
your graduated cylinder. Place the coiled copper in the beaker and then
place the beaker on a hot plate.
Warm gently. Periodically, knock the silver from the coil with a clean
stirring rod.
The speed at which the reaction occurs depends on the
concentration of silver ion. It is necessary to test the mixture in the
beaker to determine when the reaction is completed. Silver ion and
chloride ion combine to form an insoluble precipitate. Place two drops of
the unknown solution in a clean test tube and add 1 mL of water. Add
two drops of 6M HCl solution. The precipitate indicates the presence of
the silver ion:
Ag+ + Cl– → AgCl(s)
2
After 15 minute time intervals, test the reacting solution in the
beaker for the presence of silver ion according to the procedure outlined
above. If you do not see a precipitate or if the test shows a slightly cloudy
appearance, assume the reaction is complete.
When the reaction is complete, carefully tap off any silver stuck to
the copper wire and remove the wire. Unwind the copper coil and clean
and dry the metal. Weigh the copper wire and find the weight of copper
reacted.
Withdraw a 5 mL sample of solution from the beaker and observe
the color. The presence of Cu+2 ion may be confirmed by the formation of
a deep blue complex when ammonia is present. Add ammonia (labeled as
6M NH4OH or 6M NH3) to the 5 mL sample. Observe and record the color
of the solution. The reaction is:
Cu+2 + 6 NH3 → Cu(NH3)6+2
light blue
deep blue
Pour off the remaining solution from the beaker. Try not to lose
any silver particles. Wash the silver in the beaker three times with 25 mL
portions of distilled water, pouring off the wash water carefully. Transfer
the silver to a clean, dry evaporating dish. Pour off as much water from
the evaporating dish as possible, then dry the silver by heating the dish
on a hot plate. Cool the dish when dry and weigh to determine the
amount of silver present. Place the silver in a collecting jar.
Data:
Unknown silver code or number
Weight of copper before reaction
Weight of copper after reaction
Weight of copper reacted
Results of Ag+ ion Test:
a. After 15 minutes
b. After 30 minutes
c. After 45 minutes
g
g
g
3
Results of Cu+2 ion Test:
a. Color of solution before adding NH3
b. Color of solution after adding NH3
Weight of evaporating dish and silver
Weight of dry, empty evaporating dish
Weight of elemental silver formed
g
g
g
Calculations:
Use the weights of copper and silver reacted to make the following
calculations:
1.
Calculate the number of moles of copper reacted.
2.
Find the number of moles of silver reacted.
3.
Calculate the ratio of moles of silver to moles of copper.
4.
Find the concentration of silver in the solution in terms of the
number of moles of silver per liter of solution.
4
Questions:
1.
Balance the equation below:
Al
+
Cu(NO3)2 →
Al(NO3)3
+
Cu
2.
How many grams of copper are in 50.0 mL of a solution containing
0.500 moles of Cu+2 ion per liter?
3.
If silver sells for $11.45 per ounce, what is the value (dollar
amount) of 50.0 mL of a solution containing 12.0 grams of Ag+ ion
per liter?