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Understanding Moles
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| Subject
Area |
General Chemistry | ||||||||||||||||||||||||||||||||||||||||||||||||
| Age
or Grade |
Chemistry I: Grade 10 or 11 | ||||||||||||||||||||||||||||||||||||||||||||||||
| Estimated
Length |
One 45 minute class period | ||||||||||||||||||||||||||||||||||||||||||||||||
| Prerequisite
knowledge/skills |
Students will have already been introduced to the mole concept as part of the unit on chemical reactions and stoichiometry. | ||||||||||||||||||||||||||||||||||||||||||||||||
| Description
of New Content |
This lesson will serve to reinforce the concepts introduced in earlier lectures for those students having difficulty grasping the ideas in this simple, yet often confusing, topic. | ||||||||||||||||||||||||||||||||||||||||||||||||
| Goals |
Students' conception and understanding of the mole concept will be enhanced. | ||||||||||||||||||||||||||||||||||||||||||||||||
| Materials
Needed |
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|
Procedure
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Opener Begin with the concept of dozen. Students seem to easily forget that a mole is simply a counting number, just like a score, a pair, and a dozen. Explain that a mole, or 6.02 x 1023, is just a convenient number for talking about atoms. Ask the students why they think it is that we don't talk about atoms and molecules in terms of dozens or scores. The students should readily realize that atoms and molecules are too tiny and lightweight to be able to manipulate a few dozen on a lab bench or weigh them on a balance. In order to refer to quantities of atoms and molecules that are useful, we must talk about billions of billions. Development One of the reasons students tend to be easily confused by the mole concept is the seemingly arbitrary nature of the number itself, which tends to enable students to forget that it is simply a counting number. Ask the students where they think Avogadro's number comes from. Ask what they think may be the link between the atomic mass unit (amu) and the gram. Guide the students to the realization that Avogadro's number is the conversion factor between amu and grams; i.e.: there are6.02 x 10 23 amu in one gram. Thus, one mole of protons (or neutrons) weighs approximately one gram. Since the mass of all elements and molecules is, for all practical purposes, due entirely to the mass of the constituent protons and neutrons, speaking of atoms and molecules in terms of molar quantities enables us to move readily from talking about single atoms or molecules to gram quantities of atoms and molecules. In fact, one mole of a substance gives us, in grams, the mass, in atomic units, of one atom or molecule. Closure The final aspect that seems to cause confusion for students is that molar quantities of different substances have different masses; i.e.: one mole of a substance doesn't necessarily weigh the same as one mole of another substance. This seems to stem from relapse into forgetting that a mole is simply a quantity and nothing more.To solidify this realization, ask the students what weighs more, a mole of elephants or a mole of ping pong balls. Invariably, having just recently been introduced to the mole concept, some students will answer that the two are the same weight. Guide the students by returning to more familiar quantities. Explain that the question proposed is analagous to asking whether or not a dozen elephants weighs more than a dozen ping pong balls. Explain that, for the same reason that a dozen elephants weighs more than a dozen ping pong balls, a mole of CO2 molecules weighs more than a mole of H2. |
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| Evaluation |
The students are to complete the
following worksheet for homework, so as to reinforce the concepts
covered, as well as provide practice in calcuations involving
interconversion between mass and moles. Understanding Moles 1. How many eggs would you have if you had:
2. Why do you think we talk about eggs in terms of relatively small numbers (dozens) and atoms and molecules in terms of moles? Do you think it would be easy to work with or measure the mass of a dozen atoms or molecules? 3. What is the relationship between atomic mass units (amu) and moles? How many amu are in one gram? 4. Find the atomic mass and molar mass of each element.
5. Find the molecular mass and molar mass of each molecule
6. How many moles of molecules are there in 32 g of O2? How many molecules is this? 7. How many moles of molecules are there in 156 g of C6H6? How many molecules is this? 8. How many grams are there in 3 moles of C3H6O? How many molecules is this? 9. How many molecules are there in 44 g of C4H8O2? 10. How many molecules are there in 56 g of N2? 11. How much do 1.2 x 1024 molecules of O2 weigh? 12. How much do 1.2 x
1024 molecules of CO2 weigh? |
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| Extensions |
see the Lesson on Why Diet Coke Floats | ||||||||||||||||||||||||||||||||||||||||||||||||
| References |