Lab 12 Determination of the Empirical Formula of Silver Oxide
Honors Chemistry
Background
The composition of a chemical compound—what it is made of—can be described in at least three different
ways. The percent composition fixes the percent by mass of each element in the compound and is the
simplest way experimentally to describe the composition of a substance. Calcium carbonate, for example,
contains calcium, carbon and oxygen. It is present in eggshells and seashells, chalk, and limestone. The
mass percentage is 40 % calcium, 12% carbon, and 48% oxygen.
However, in terms of understanding how elements come together to make a new compound, it is more
interesting and more informative to know how many atoms of each kind of element combine with one
another. The empirical formula describes the composition of a compound in terms of the simplest wholenumber ratio of atoms in a compound and does not necessarily represent the actual number of atoms in a
molecule for formula unit.
The molecular formula of a compound tells us the actual number of atoms in a single molecule of a
compound.
Experimental Overview
In this experiment, the percent composition and empirical formula of silver oxide will be determined.
Silver oxide decomposes to silver metal and oxygen when strongly heated. Heating silver oxide causes the
oxygen to be driven off, leaving only the silver metal behind. According to the law of conservation of
mass, the total mass of the products of a chemical reaction must equal the mass of the reactants. In the
case of the decomposition of silver oxide, the following equation must be true:
Mass of silver oxide = Mass of silver metal + Mass of oxygen
If both the initial mass of silver oxide and the final mass of the silver metal are measured, the decrease in
mass must correspond to the mass of oxygen that combined with the silver. The percent composition and
empirical formula of silver oxide can then be calculated, based on combining the ratios of silver and
oxygen in the reaction.
PreLab Questions
Unbalanced equation:
Fe(s) + O2(g )
Fe2O3(s)
1. Balance the above equation
2. Use 0.055 g iron to covert the mass of iron used to moles. Use factor label.
3. Calculate the mass of oxygen that reacted with the iron.
4. Use the molar mass of oxygen to convert the mass of oxygen, O2, reacted to moles. Use factor label.
5. Use the moles of O2 and calculate the moles of O. (1mole O2 = 2 moles of O)
6. Use the ration between the number of moles of iron and number of moles of oxygen, O not O2) to
calculate the empirical formula for iron oxide. (Show your work). Remember that fractions of moles of
atoms do not exist; therefore the ratio should be simplified by multiplying all of the moles by a
constant to give a whole number ratio. Example: HO1/2, multiple by moles by 2 to achieve whole
numbers H2O.
Materials
Silver oxide samples, ~0.5 g
Crucible and crucible lid, 15 or 30 mL
Crucible tongs
Bunsen burner
Ring stand and ring clamp
Balance (0.001 gram precision)
Clay pipestem triangle
Ceramic plate
Wash bottle and water
Watch glass (optional)
Safety Precautions
Silver oxide is slightly toxic. Handle the crucible and lid only with tongs. Remember that a hot crucible
looks exactly like a cold one. Wear chemical goggles and gloves. Wash hands thoroughly with soap and
water before leaving the laboratory.
Procedure
1. Set up a Terrill burner nest to the white hoods, on a ring stand beneath a ring clamp holding a clay
triangle. Do not light the Terrill burner.
2. Adjust the height of the ring clamp so that the bottom of the crucible sitting in the clay triangle is
about 1 cm above the burner. This will ensure that the crucible will be in the hottest part of the flame
when the Terrill burner is lit.
3. Measure the mass of a clean, empty crucible to the nearest 0.001 gram. Record the mass in the data
table.
4. Add approximately 0.5 grams (see visual example, do not mass) of silver oxide sample to the crucible.
Measure the combined mass of the crucible and silver oxide to the nearest 0.001 gram. Record the
mass in the data table.
5. Place the crucible on the clay triangle. Light the Terrill burner and slowly heat the crucible by brushing
the bottom of the crucible with the flame for 2-3 minutes.
6. Place the burner on the ring stand and
gently heat the crucible for an additional 10 minutes.
Heating the crucible gently as in steps 5 and 6 will avoid splattering the sample.
7. After 10 minutes of gently heating the sample, place the burner directly under the crucible. Heat the
crucible with the most intense part of the flame for 10 minutes. Caution: do not inhale the smoke. Do
not lean over the crucible.
8. After 10 minutes of intense heating turn off the gas source.
9. Allow the crucible to cool for at least 10 minutes.
10. Measure the combined mass of the crucible and the silver metal product. Record the mass in the data
table.
11. If time permits dump the entire contents of the crucible onto a watch glass and note the appearance of
the product. Is any un-reacted silver oxide still present? Record all observations in the data table.
12. Dispose of cooled sample in the trash can
13. Repeat steps 1-12 for trials # 2 and # 3
Data and Calculations Tables
Data Table
Trial #1
Mass Crucible (g)
Mass Crucible +
Silver oxide (g)
Mass of Silver oxide
(g)
Mass of crucible and
silver (g) after
heating
Trial #2
Mass of silver (g)
Moles of silver
Mass of oxygen (g)
Moles of oxygen
Appearance of
Product
Post Lab Calculations – complete questions 1-9 for each trail
1. Show your calculations to determine the mass of silver oxide that was used, the mass of silver after heating and the
mass of oxygen produced. Enter you answers into the Data Table.
2. Calculate the percent composition of silver in the silver oxide. Show your work.
3. Calculate the percent composition of oxygen in the silver oxide, Show your work.
4. Use factor label to convert grams of silver to moles of silver.
5. Use factor label to convert grams of oxygen to moles of oxygen.
6. Calculate the empirical formula for silver oxide based on your lab data. Show your work.
7. Calculate the percent composition of silver in one mole of silver oxide using your empirical formula. This is the
theoretical percent composition.
8. Calculate the percent composition of silver in your silver oxide compound. (Mass silver / Mass silver oxide times
100%). This is your experimental percent composition.
9. Calculate your percent error: [ ITheor – Exper.I / Theor ] ] x 100% Show your work.
10. Discuss two possible sources of error. Example: What did you do specifically that could have altered the result?
Calculating errors, human error and incorrect mass measurement are NOT considered sources of errors.

Discuss error sources such as:
o Fingerprints on crucible at initial weighing.
o Small amount of silver oxide remaining.
o Crucible no at constant weight before starting the experiment.
Determination of the Empirical Formula of Silver Oxide
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1 pt Rubric included with lab
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5 pts Neat, answers include question
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8 pts Lab physically completed within one week of assigned time.
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2pts Lab Set Up: Lab is written on the right side of the composition book only. Lab is included in the table of
contents
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2pts Pre-Lab 1: Clearly labeled conversion for iron from grams to moles. Answer provided with the correct
number of significant figures, units and species.
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2pts: Pre-Lab 2: Clearly labeled calculation to determine the mass of oxygen. Answer provided with the correct
number of significant figures, units and species.
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2pts Pre-Lab3: Clearly labeled conversion for oxygen from moles to grams. Answer provided with the correct
number of significant figures, units and species.
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3pts Pre-Lab4: Clearly labeled calculations to show the determination of the empirical formula of iron oxide.
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5pts Data Table 1: completely filled out with calculations to determine the mass of sliver oxide that was used,
the mass of silver after heating and the mass of oxygen produced.
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2pts Post-Lab 2: Clearly labeled calculations showing the percentage of silver in the compound. Answer is
recorded in Data Table .
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2pts Post-Lab 3: Clearly labeled calculations showing the percentage of oxygen in the compound. Answer is
recorded in Data Table .
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2pts Post-Lab 4: Clearly labeled conversion of grams of silver to moles. Answer provided with the correct
number of significant figures, units and species. Answer is recorded in Data Table 2.
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2pts Post-Lab 5: Clearly labeled calculation to show the conversion of grams of oxygen to moles of oxygen.
Answers are recorded in Data Table.
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3pts Post-Lab6: Calculate the empirical formula for silver oxide based on your lab data. Show your work.
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3pts Post-Lab7. Calculate the percent composition of silver in one mole of silver oxide using your empirical
formula. This is the theoretical percent composition.
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3pts Post-Lab8: Calculate the percent composition of silver in your silver oxide compound. (Mass silver / Mass
silver oxide times 100%). This is your experimental percent composition
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2pts Post-Lab9: Calculate your percent error: [ ITheor – Exper.I / TheorI ] x 100% Show your work.
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2pts Post-Lab10 Discuss two possible sources of error. Example: What did you do specifically that could have
altered the result? Calculating errors, human error and incorrect mass measurement are NOT considered
sources of errors.