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 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:

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 canthen be calculated, based on combining the ratios of silver and oxygen in the reaction.

PreLab Questions

Unbalanced equation: Fe(s) + O2(g ) Fe2O3(s)

Before you begin Balance the above equation

1. Use 0.0550 g iron to covert the mass of iron used to moles. Use factor label.

  1. Use the moles of iron, and the balanced equation, to determine the number of moles of O. (1 mol O2 = 2 mol O)
  1. Determining the empirical formula:

Use the ratio between the number of moles of iron and the number of moles of oxygen to calculate the empirical formula for iron oxide. To do this, divide both mole values by the smallest number of moles. This will force your mol ratio to be the smallest whole number ratio. (If your ratio is 1:1.5, multiply both mole by 2) (If your mole ratio is 1: 1.33 multiple both moles my 3).

  1. 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

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.
  1. 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.
  1. Measure the mass of a clean, empty crucible to the nearest 0.001 gram. Record the mass in the data table.
  1. 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.
  1. 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.
  1. 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.
  1. 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.
  1. After 10 minutes of intense heating turn off the gas source.
  1. Allow the crucible to cool for at least 10 minutes.
  1. Measure the combined mass of the crucible and the silver metal product. Record the mass in the data table.
  1. 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.
  1. Dispose of cooled sample in the trash can
  1. Repeat steps 1-12 for trials # 2 and # 3

Data and Calculations Tables

Data Table

Trial #1 / Trial #2
Mass Crucible (g)
Mass Crucible + Silver oxide (g)
Mass of Silver oxide (g)
Mass of crucible and silver (g) after heating
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.
  1. Calculate the percent composition of silver in the silver oxide. Show your work.
  1. Calculate the percent composition of oxygen in the silver oxide, Show your work.
  1. Use factor label to convert grams of silver to moles of silver.
  1. Use factor label to convert grams of oxygen to moles of oxygen.
  1. Calculate the empirical formula for silver oxide based on your lab data. Show your work.
  1. Calculate the percent composition of silver in one mole of silver oxide using your empirical formula. This is the theoretical percent composition.
  1. Calculate the percent composition of silver in your silver oxide compound. (Mass silver / Mass silver oxide times 100%). This is your experimental percent composition.
  1. Calculate your percent error: [ ITheor – Exper.I / Theor ] ] x 100% Show your work.
  1. 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:
  • Fingerprints on crucible at initial weighing.
  • Small amount of silver oxide remaining.
  • Crucible no at constant weight before starting the experiment.

Determination of the Empirical Formula of Silver Oxide

_____ 1 pt Rubric included with lab

_____ 5 pts Neat, answers include question

_____ 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.