Unit 5 – Counting Particles Too Small to See
Sequence
1. Discussion: counting by weighing, Avogadro’s Hypothesis
2. Activity: Relative mass of beans, data collection and evaluation
3. Post-lab discussion – definition of the mole, molar mass as relative mass of elements, worksheet 1
4. Size of a mole worksheet
5. Use of molar mass as conversion factor between “how much” and “how many”, begin Empirical Formula
lab, worksheet 2 – p1
6. Finish Empirical Formula lab, evaluation of data, comparison of class results
7. Post-lab discussion, worksheet 2 - p2
8. Empirical and molecular formulas – quiz on mass  mole conversions
9. Percent composition, worksheet 3
10. Begin nail lab – unit review
11. Part 2 of nail lab - finish unit review
12. Unit test
INTRODUCTION
Imagine a large bag of Styrofoam packing peanuts …..how easy would it be to count all of them? Let’s
say that 100 peanuts had a mass of 5.5 g. Describe a method to determine how many peanuts are in the
bag.
This was the challenge that early chemists faced when they wanted to get some sense of how many
atoms or molecules were present in a sample they were investigating. Way back in Unit 1 we found that
iron was more dense than aluminum. Two possible models arose to account for this difference.
A. The masses of Al and Fe atoms are about the same, but there are more
atoms of Fe than atoms of Al in each cm3 sample.
B. One cm3 samples of Fe and Al contain about the same number of atoms,
but the Fe atoms are more massive.
A third possibility – that both the size and the mass of the atoms of these two elements were different
– also came up. At the time, we did not have enough evidence to make a decision about these possible
models.
While the reason for density variation between particle types is difficult to determine for liquids and
solids, a conclusion can be reached more easily for gases due to the fact that particles in a gas are
widely spaced. This means that particle size does not have an effect on the volume that a given
number of gaseous particles occupy. Review the experimental evidence of Avogadro’s Hypothesis. This
hypothesis not only allows us to deduce the numbers of molecules that react with one another based on
volumes, but also enables us to determine the relative masses of elements and compounds.
Imagine that the beans in the Relative Mass Lab are enlarged particles of various gases…
©Modeling Instruction – AMTA 2012
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U5-Counting v3.1
Chemistry Learning Plan for 1/30 and 1/31:
1.
Obtain a copy of the following documents:
a. U5 Learning Targets
b. U5 Introductory Reading
c. U5 Lab: Relative Mass
2.
Divide the class up into working groups of 2 or 3 students. Each group needs to be working in their
own space. Please do not “bunch up” with another group for these activities.
3. Enter a U5 Introduction Section in your lab NB. Read “Introductory Reading” with your group.
Record the main points of the reading in your lab NB.
4. Read the introduction to the Relative Mass Lab and work with your group to collect and record data.
I will have designated which balances you need to use….please make sure they are stored under my
desk before you leave the classroom.
5. Work through the calculations with your lab group.
6. Discuss the Post-lab questions with your group and complete them in your lab NB in complete
sentences!
7. Make a WB to show your RESULTS….make sure that you can discuss your results.
8. Start working on U5 WS1.
©Modeling Instruction – AMTA 2012
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U5-Counting v3.1