Laboratory Activity 2: Teacher Notes

Continued

Post-Laboratory Discussion

  1. Start by collecting class data for this activity. By posting results, a trend should emerge, revealing consistent results around 60% Mg and 40% O. Discuss how these results relate to the Law of Definite Composition.
  2. Questioning about high or low percent values would be a natural follow up. Include as many students as possible in the discussion.
  3. Ask such questions as "Would the amount of Mg used affect the results?" Also probe on the usefulness of knowing the percent composition of a compound. Propose that the students are chemists in charge of producing fertilizer for a company. Ask them to explain how this activity has a connection to their success in this company.

Extensions

  1. A key part of this laboratory activity is to use data collected for the class (or several classes, if desired) to illustrate the importance of the mole concept and the determination of empirical formulas.
  2. Ask students to calculate moles Mg and moles O from their experimental data. Plot moles Mg vs. moles O for the class. This usually gives a beautiful graph with the expected slope of about 1. Feel free to discard individual data points that are obviously far from the trend. These are decisions that scientists with guidance from statistics can make. Explain to students that many experimental trials are often needed to produce reliable results. With thousands of trials, data points far removed from the trend would have little effect on the overall statistical analysis of results.
  3. This graph verifies the formula for magnesium oxide (MgO) that students used to calculate their theoretical mass percent values for Mg and O. Explain how the slope of the graph can be used to determine the empirical formula. (Alternately, extrapolate the graph to 1 mol Mg and show that the corresponding value for O is also 1 mol.)
  4. Have students investigate common substances made up of the same elements that differ in their percent composition by mass, such as H2 O2and H2 O. Establish differences in their properties and uses.
  5. Invite students to investigate a major industrial chemical process such as the Haber, Ostwald, Contact, or Hall process to identify major reactants and to reinforce the notion that knowledge of amounts of required substances is a component of being a successful industrial chemist or engineer.

Assessing Laboratory Learning

Laboratory Practical

Provide students with sample data on another compound composed of a metal and a nonmetal. Have them perform calculations to identify the compound by comparing their results to a list of "known" compound possibilities.

Examination Questions

  1. Three metals are allowed to react as completely as possible to form their oxides. The possible oxides are Cu 2O, CuO, and ZnO. Which would have the highest percent oxygen by mass? [CuO]
  2. If you were given 45.0 g of aluminum, what minimum mass of oxygen would you need to allow the aluminum to react completely to form Al2O3 ? [40.0 g O2]

Other Laboratory Activities

  1. A good activity is ask students to weigh a 1-in square of Al foil and calculate the number of such squares that must be stacked to provide 1 mol Al. A 1-in square of aluminum foil has almost exactly 108 Al atoms on each edge. Therefore, a 1-in cube of Al would approximate one mole (approximately 1024) Al atoms.
  2. Determining the percent water in a hydrate. This laboratory activity can also be used to illustrate the determination of percent composition. The calculations are not as clear to beginning students as the percent of an element in a compound (as in Laboratory Activity 2 ), but the results are usually good. (A typical laboratory experiment is given by Slowinski, 1985.)
  3. Determining Avogadro's number (and molecular size) from measurements on a unimolecular layer of stearic or oleic acid. This activity provides a way to obtain an approximate value for Avogadro's number. The assumptions needed are usually beyond the grasp of beginning students. However, if you do not dwell on the assumptions, students can understand the calculations reasonably well and will obtain a value of approximately the correct order of magnitude (1022 to 1024) for Avogadro's number. Corwin (1988) describes an experimental procedure.
  4. Determining an empirical formula from the reaction of Fe with CuSO4 . This laboratory activity gives good results. A modified version of the activity is given in the Stoichiometry module. The data can be used to calculate the empirical formula for CuSO4(mole ratio of Cu2+ to SO4 2- ). The experimental procedure is the same.
  5. NOTE: We strongly discourage using the experimental determination of the empirical formula of a sulfide of copper, which you may find in older laboratory manuals. Many individuals are extremely sensitive to the SO2formed in the activity. Persons with asthma can have severe reactions to SO2 .
    TABLE OF CONTENTS TOPIC OVERVIEW CONCEPT/SKILLS DEVELOPMENT LINKS/CONNECTIONS EXTENSIONS