Determining the Formula of a Compound
Teacher Guide
Materials
Item Description
Silver nitrate
Copper wire, 22 gauge
Copper wire, 16 gauge
Beaker, 100-mL
Beaker, 250-mL
Test tube, 20x150mm
Utility clamp
Funnel
Filter paper
Ring stand
Ring, iron
Parafilm
Wash bottle
Stirring rod
Gloves, latex, medium
Drying oven (optional)
Vendor
Flinn Scientific
Flinn Scientific
Flinn Scientific
Flinn Scientific
Flinn Scientific
Flinn Scientific
Flinn Scientific
Flinn Scientific
Flinn Scientific
Flinn Scientific
Flinn Scientific
Flinn Scientific
Flinn Scientific
Flinn Scientific
Flinn Scientific
Flinn Scientific
Catalog #
S0434
C0151
C0147
GP1010
GP1020
GP6030
AP1034
AP3201
AP3104
AP8228
AP1320
AP1501
AP5343
GP5075
AP4428
AP1193
Student Learning Goals
 Recognize that atoms are neither created nor destroyed in chemical reactions, merely
rearranged.
 Write balanced equations representing chemical reactions from word descriptions or
given formulas of reactants and products.
NOTES
Silver nitrate is corrosive; students must wear goggles at all times during this activity.
There are advantages to using the reaction between AgNO3 and Cu as the basis for analyzing
the mass and molar relationships of the products and reactants. This reaction gives a good yield
and the silver metal produced is easily filtered and rinsed; unlike other gravimetric analysis labs
where precipitates clog the pores of the filter paper and make the filtering and drying process
painfully slow. Students can also be introduced to gravimetric analysis techniques in an
environment where mistakes are less critical. A drying oven is recommended but not necessary,
the filter paper, silver metal, and Cu wire can be air dried with good results. Another advantage
is the mole ratio of Ag to Cu is 2 to 1; most other metal replacement reactions used for this
activity give a mole ratio of 1 to 1, which may lead students to conclude erroneously that all
1
mole ratios are 1 to 1. The blue color of the Cu(NO3)2 produced and the formation of the silver
metal act as pronounced visual cues for students that a chemical reaction is taking place
One of the disadvantages of this activity is the high cost of AgNO3. Another is the reaction of
AgNO3 with skin and clothing to produce dark stains. Latex gloves can be issued to students to
prevent getting the AgNO3 on their hands. AgNO3 stains can be removed from clothing using
Spray-N-Wash®. Stains on skin can be removed using Erada-Stain® cream available from Flinn
Scientific (AP7330). This activity can be done without incurring the cost of latex gloves if
laboratory hygiene is stressed to students before and during the activity.
The most common causes of erroneous results in this activity:
 Students weighing the 100-mL beaker at the beginning of the activity and then using a
different 100-mL beaker as a container for the silver metal and filter paper.
Many students aren’t aware of the mass differences between the same sized beakers or
filter paper.
 Not dissolving the AgNO3 completely in the distilled water before adding the Cu wire.
 Using tap water instead of distilled to dissolve the AgNO3. Have students check their
AgNO3 solutions before adding it to the test tube; the solutions should be clear, if it is
cloudy they should discard in a waste container and make a fresh solution.
 Weighing another lab group’s Ag/filter paper/beaker combination from the drying
oven.
The following questions can be substituted for #7 in the CALCULATIONS AND RESULTS section
depending on the level of student performance.
7. Using the class results, write whole-number coefficients in the following equation and give the
formula for the other product of this reaction:
__1__mole Cu(solid) + ____mole(s) AgNO3 (solution) ---->___mole(s) Ag
(solid)
+ __ mole(s) _______(solution)
7. Using the class data for the ratio of moles Ag/ 1mole Cu, draw a diagram that represents how the
atoms in this chemical reaction rearrange.
PRE-LAB DISCUSSION
1) Students must wear goggles throughout this activity. Stress the need for keeping the lab
area clean. Clean up spills immediately and wipe down the lab station when done.
2) Actively present the procedure using the equipment the students will use. An important
question for students to answer is, “How will you know when the filter paper/silver/beaker and
Cu wire are dry?”
Lead students in a discussion about the need for successive weighings and stabilization of the
mass readings of each of these components.
2
After students have been oriented to the procedures of the activity, the teacher can familiarize
students with the calculations needed to complete the laboratory activity by displaying the
following data set and questions below:
Mass of labeled beaker
52.32 g
Mass of weigh tray
2.24 g
Mass of weigh tray & AgNO3
5.24 g
Mass of Al wire before rxn.
3.50g
Mass of filter paper
0.92 g
Mass of Al wire after rxn
(Day 1)
3.41 g
Mass of Al wire after rxn
(Day 2)
3.36 g
Mass of labeled beaker, filter paper & Ag (Day 1)
54.48 g
Mass of labeled beaker, filter paper & Ag (Day 2)
54.53 g
a)
b)
c)
d)
Calculate the number of moles of aluminum (Al) wire that reacted.
Calculate the number of moles of silver (Ag) that was produced.
Calculate ratio of moles of Ag/ 1 mole of Al.
Using the ratio you calculated in #3, complete the following chemical equation,
including the formula for aluminum nitrate.
___Al + ___AgNO3 ----> ___Ag + ___ ________
Problem solution:
a)
Mass of Al wire assembly before reaction
Average mass of Al wire after reaction
Mass of Al reacted
1 𝑚𝑜𝑙 𝐴𝑙
0.11 𝑔 𝐴𝑙 (
27.0 𝑔
3.50 g
-3.39 g
0.11 g
) = 0.0041 𝑚𝑜𝑙 𝐴𝑙 𝑟𝑒𝑎𝑐𝑡𝑒𝑑
b)
Average mass of 100-mL beaker, filter paper, Ag
Mass of labeled 100-mL beaker
Mass of filter paper
Mass of Ag metal produced
1 𝑚𝑜𝑙 𝐴𝑔
1.27 𝑔 𝐴𝑔 (
107.9 𝑔
54.51 g
-52.32 g
-0.92 g
1.27 g
) = 0.0118 𝑚𝑜𝑙 𝐴𝑔 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑
c) 0.0118 mol Ag/0.0041 mol Al = 2.9 mol Ag/ 1 mol Al
d) Al + 3 AgNO3 ----> 3Ag + AlN3O9 or Al(NO3)3
3
3) There is no data table outlined in the activity instructions. Depending on the level of
students, they can create their own as a way of reviewing the procedure or the following data
table can be displayed and discussed.
Mass of weigh tray
Mass of weigh tray and AgNO3
Mass of labeled 100-mL beaker
Mass of Cu wire assembly before reaction
Mass of filter paper
Mass of Cu wire after reaction
Mass of 100-mL beaker, filter paper, Ag
Mass of Cu wire after reaction
Mass of 100-mL beaker, filter paper, Ag
Mass of Cu wire after reaction
Mass of 100-mL beaker, filter paper, Ag
1st weigh
1st weigh
2 nd weigh
2 nd weigh
3 rd weigh
3 rd weigh
Diagram of student set up showing the copper wire assembly before reacting with the AgNO 3:
See Flinn Scientific disposal method #11b for AgNO3 solid or solution and #26b for the Cu(NO3)2
solution produced.
4
Sample Student Data
Mass of weigh tray
Mass of weigh tray and AgNO3
Mass of labeled 100-mL beaker
Mass of Cu wire assembly before reaction
Mass of filter paper
Mass of Cu wire after reaction
1st weigh
Mass of 100-mL beaker, filter paper, Ag
1st weigh
Mass of Cu wire after reaction
2nd weigh
Mass of 100-mL beaker, filter paper, Ag
2nd weigh
Mass of Cu wire after reaction
3rd weigh
Mass of 100-mL beaker, filter paper, Ag
3rd weigh
2.13 g
5.13 g
52.32 g
2.86 g
1.06 g
2.30 g
55.46
2.31 g
55.31
Not needed
55.30
CALCULATIONS AND RESULTS
1.
Mass of Cu wire assembly before reaction
Average mass of Cu wire after reaction
Mass of Cu reacted
2.
2.86 g
-2.31 g
0.55 g
1 𝑚𝑜𝑙 𝐶𝑢
0.55 𝑔 𝐶𝑢 ( 63.55 𝑔 ) = 0.0087 𝑚𝑜𝑙 𝐶𝑢 𝑟𝑒𝑎𝑐𝑡𝑒𝑑
3.
Average mass of 100-mL beaker, filter paper, Ag
Mass of labeled 100-mL beaker
Mass of filter paper
Mass of Ag metal produced
55.31 g
-52.32 g
-1.06 g
1.93 g
4.
1 𝑚𝑜𝑙 𝐴𝑔
1.93 𝑔 𝐴𝑔 (
107.9 𝑔
) = 0.018 𝑚𝑜𝑙 𝐴𝑔 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑
5. 0.018 mol Ag/0.0087 mol Cu = 2.07 mol Ag/ 1 mol Cu
6. Student ratios
5
7. Since the ratio of moles of Ag to moles of Cu is 2 to 1:
__1__mole Cu(solid) + ____mole(s) AgNO3 (solution) ---->__2_mole(s) Ag (solid) + __ mole(s) _______(solution)
If 2 moles of Ag are produced for every mole of Cu used, then 2 moles of AgNO3 must be
reacted if atoms are conserved:
__1__mole Cu(solid) + __2__mole(s) AgNO3 (solution) ---->__2_mole(s) Ag (solid) + __ mole(s) _______(solution)
If atoms are conserved and there is only one other product, the compound produced must be
composed of 1 mole of Cu atoms chemically combined with 2 moles of nitrogen atoms and 6
moles of oxygen atoms:
__1__mole Cu(solid) + __2__mole(s) AgNO3 (solution) ---->__2_mole(s) Ag (solid) + _1_ mole CuN2O6 or Cu(NO3)2 (solution)
Therefore:
Cu + 2 AgNO3 ----> 2Ag + CuN2O6 or Cu(NO3)2
8.
(a) The mass of the water clinging to wet Ag metal would give an experimental value for the Ag
that is greater than the actual mass of the Ag alone. This makes the experimental ratio of moles
of Ag to moles of Cu higher than the actual ratio.
(b) The mass of the copper wire assembly that breaks off doesn’t react to produce silver. This
causes the experimental value for the mass of Cu that reacted to be greater than the actual
mass. This results in an error that makes the ratio moles of Ag to moles of Cu lower than the
actual value.
9. CuN2O6 or Cu(NO3)2
6