TITLE: The Mole as a large number
or
An exercise in Dimensional Analysis
AUTHOR: Dick Fargo, Clearwater Valley High School, Kooskia, Idaho
GRADE LEVEL/SUBJECT: 9-12
OVERVIEW: With a medicine dropper, let one drop of water fall in front of
class. Pose the question, "If this water molecule evaporates at the rate of
one million (10^6) molecules per second, how long will it take for the
molecule to be gone?"
PURPOSE: This may be used either as a synthesis project where students
are expected to bring together possessed knowledge to solve a problem, or
as a demonstration either for the concept of large numbers or as a fairly
complex example of what can be done with dimensional analysis
(factor/label or whatever you may call it).
OBJECTIVES: By participating in this activity the students will:
1. Be required to make "proveable estimates".
2. Review important basic facts of chemistry.
3. Use concepts of metric system.
4. Consider large numbers with "real world" application.
5. Be exposed to the power of dimensional analysis in solving a complex
problem.
6. Say "Wow!"
RESOURCES/MATERIALS: None other than the eyedropper are required,
however you may wish to have a metric ruler, a chemisty book and a
balance close by in case.
ACTIVITIES AND PROCEDURES: This activity may be used either as a
"class or small group synthesis project" or as a "class participation
demonstration". I usually work it in toward the end of the dimensional
analysis unit. It can also be used as a test question for a fairly quick
group.
Use a medicine dropper to drop one drop of water on a surface where all
students may see it. Then pose the problem, "If one million molecules
evaporate each second, how long will it take the drop to completely
evaporate?"
As a demonstration, the steps for solution would be:
1. Estimate the volume of a drop of water. It is quite easy to drop a drop
of water on a balance but the discussion and the logic of estimating is
interesting. (A common estimate is that it would take four drops side by
side to measure one centimeter, thus 64 drops to the cubic centimeter.
Not very accurate, but good enough. You may be lucky enough to have those
who insist on weighing the water)
2. Layout the general format for D.A., start with drop of water. Call for
estimates of proper ending unit, seconds??, minutes??, hours??
3. Fill in canceling units: Drops of water to ccm of water, ccm of water
to grams of water, grams of water to mole, mole to Avogadro's number. of
molecules, one million molecules to one second (the evaporation rate),
sixty seconds to one minute, etc.
4. In a demonstration, this will naturally lead to solving for the number
of molecules which evaporate from a drop of water at room temperature.
CULMINATION ACTIVITY: Ask each student to design a problem of
somewhat this type.
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John Kurilecjmk@ofcn.org