Laboratory Activity: Teacher Notes

Activity 1: Understanding the Mole

Major Chemical Concept

Molar masses are derived from relative masses (see SI definition of mole in Content in a Nutshell). Molar masses of different substances have different masses and volumes. Each relative mass contains the same number of particles.

Level

This activity may be used with first-year high school chemistry students in basic, general, or honors levels.

Expected Student Background

Students should know how to use a balance. They should have a scientific calculator and be able to use and understand the calculator's exponential function. It would be helpful if students were acquainted with the concepts of significant figures, and knew that the last digit is an estimated number and that some variation is expected in this value.

Time

This is a two-day activity, assuming 45 to 55-min class periods. Part I is completed on the first day. Part II and Part III are done on the second day. Students turn in data tables, calculations, and answers to questions at the close of the second day.

Safety

1. No safety concerns are associated with this activity; students do not need to wear protective goggles, unless strict school regulations require them whenever students are at the laboratory bench.
2. The "one mole" of elements and compounds should be covered to prevent spilling. There could be a physical hazard of slipping on beans if many of them are spilled on the floor.

Materials (For 24 students working in pairs)

Nonconsumables

• Covered containers for beans
• 12 Numbered plastic drink cups
• 48 Zip-closure bags (optional)
• 6 Beakers, 50-mL (with graduations to hold one-mole amounts of elements and water)
• Beaker, 800-mL (with graduations; to hold one mole of sucrose)

Approximate quantities of beans:

• Lentil beans, 1 bag (3 oz.)
• Lima beans, 2 large bags (2 1/4 lb.)
• Navy beans, 1 bag (8 oz.)
• Pinto beans, 1 large bag (1 1/4 lb.)

One mole of each substance in powdered or granular form, except for water:

• Water, 18 mL (18 g)
• Aluminum, 27 g
• Copper, 63.5 g
• Sucrose (table sugar), 342 g
• Carbon, 12 g Tin, 119 g
• Zinc, 65 g

Advance Preparation

First DayNumber the plastic cups. Cups may differ in mass by as much as one gram; each group should continue to use the same weighed cup. Place beans in a closable container that is easy for students to get to. If desired, approximately 120 beans of each type can be weighed into zip-closure bags (lentils - 8 g, navy beans - 208 g, pinto beans - 48 g, lima beans - 120 g per zip-closure bag). This will save student time.

Second DayPlace one mole of each of the specified elements, sucrose, and water in separate beakers. Label each one, cover each beaker with plastic wrap and put a rubber band on it. This prevents spillage and evaporation of water. Place these beakers around the laboratory. (These samples can be stored covered and reused indefinitely - except for water, which will evaporate.)

Pre-Laboratory Discussion

Draw students' attention to the Periodic Table. They will have seen the table in the classroom for several weeks-and in previous courses. They are usually curious about what it means and why it is arranged the way it is. You might ask them to spot any regularities in the chart. Among other things, they should be able to see that atomic numbers always increase across a row. Atomic masses usually increase-but not always. They also are curious about the decimal values in the atomic masses. Point out that a hydrogen atom is the lightest atom and that, for example, a gold atom weighs about 197 times more than a hydrogen atom. Students are curious how these numbers were determined, and how Avogadro's number was determined. Explain to students that by doing this activity, they will be able to understand, in part, how these things came about.

An analogy could also be discussed with students. A dozen golf balls would not have the same mass as a dozen ping pong balls. We could find the relative mass of the golf balls if we weighed a dozen ping pong balls and a dozen golf balls, and divided by the smaller mass. This would tell us not only how many more times massive a dozen golf balls were than a dozen ping pong balls, but also how many times more massive one golf ball is than one ping pong ball.

Teacher-Student Interaction

1. Move from group to group, checking to see that students understand the instructions. It is best to guide students by responding with questions that lead them to correct answers.
2. The concept of relative mass (Question 8 in Part I Calculations) may be a difficult concept for students. The concept should be clarified when the calculated value is verified experimentally (Question 10 in Part I Calculations).
3. Insure that students understand the relationship of Part II to Part I. (Since individual beans can be handled, they provide a concrete analogy. Since atoms and molecules cannot be seen, one must visualize them mentally. The calculations involving beans in Part I are analogous to those involving elements in Part II.)

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