Lab 12 Empirical Formula of Silver Oxide-2016 Version
Lab 12 Empirical Formula of Silver Oxide-2016 Version
Lab 12 Empirical Formula of Silver Oxide-2016 Version
Honors Chemistry
Background
The composition of a chemical compoundwhat it is made ofcan 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 whole-
number 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:
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.
Honors Chemistry Name__________________________________ Period___
Empirical Formula Notes:
An empirical formula represents the simplest ratio (lowest whole number ration) in which the atoms combine
to form a compound.
2. The empirical formula of a compound that has a ratio of 1 carbon to 2 hydrogen is:
3. If the empirical formula of a compound is CH2, the molecular formula could be any simple multiple of
CH2 such as:
Sample Problem A: calculate the empirical formula of a compound that contains 2.000g of calcium and 8.000 g
of bromine.
Sample Problem B: What is the empirical formula of a compound whose composition by mass is 40.00% sulfur
and 60.00% oxygen? In a problem like this even if we were given 40% and 60% you must carry through 4 sig
figs or your percent error could be off as much as 30% due to dropping sig figs. (We are not referring to O 2 gas.
Use O = 16.00 g/mol. Do not worry about oxygen being diatomic.)
Lets practice problems similar to prelab
1. Use 0.1675 g iron to covert the mass of iron used to moles. Use factor label.
2. Use the moles of iron found in question 1, and the balanced equation, to determine the number of moles
of O. (1 mol O2 = 2 mol O)
4. A compound was analyzed and found to contain 13.5 g Ca, 10.8 g O, and 0.675 g H. What is the empirical
formula of the compound?
Remember that lab data is not perfect, so many times rounding must occur if ratios other than these
are calculated.
PreLab Questions
1. Use 0.0550 g iron to covert the mass of iron used to moles. Use factor label.
3. Use the moles of iron, and the balanced equation, to determine the number of moles of O. (1 mol O 2 = 2 mol O)
6. Assume you have 40.00 g C and 6.72 grams of H and 53.28 g of O. What is the empirical formula?
Remember that lab data is not perfect, so many times rounding must occur if ratios other than these are
calculated.
Materials
Silver oxide samples, ~0.5 g Balance (0.001 gram precision)
Crucible and crucible lid, 15 or 30 mL Clay pipestem triangle
Crucible tongs Ceramic plate
Bunsen burner Wash bottle and water
Ring stand and ring clamp 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.
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 by brushing 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.
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.
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.
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.
_____ 8 pts Lab physically completed within one week of assigned time.
_____ 2pts Lab Set Up: Lab is written on the right side of the composition book only. Lab is included in the table of
contents
_____ 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.
_____ 2pts: Pre-Lab 2: Clearly labeled conversion for moles of iron to moles oxygen. Answer provided with the correct
number of significant figures, units and species.
_____ 2pts Pre-Lab3: Clearly labeled calculations to show the determination of the empirical formula of iron oxide.
_____ 3pts Pre-Lab4: Assume you have 40.00 g C and 6.72 grams of H and 53.28 g of O. What is the empirical
formula?
_____ 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.
_____ 2pts Post-Lab 2: Clearly labeled calculations showing the percentage of silver in the compound. Answer is
recorded in Data Table .
_____ 2pts Post-Lab 3: Clearly labeled calculations showing the percentage of oxygen in the compound. Answer is
recorded in Data Table .
_____ 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.
_____ 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.
_____ 3pts Post-Lab6: Calculate the empirical formula for silver oxide based on your lab data. Show your work.
_____ 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.
_____ 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
_____ 2pts Post-Lab9: Calculate your percent error: [ ITheor Exper.I / TheorI ] x 100% Show your work.
_____ 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.