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CHAPTER 2
Concept Explorations
2.25. Average Atomic Mass
Part 1: Consider the four identical spheres below, each with a mass of 2.00 g.
Calculate the average mass of a sphere in this sample.
Part 2: Now consider a sample that consists of four spheres, each with a different mass: blue mass is
2.00 g, red mass is 1.75 g, green mass is 3.00 g, and yellow mass is 1.25 g.
a.
Calculate the average mass of a sphere in this sample.
b.
How does the average mass for a sphere in this sample compare with the average mass of the
sample that consisted just of the blue spheres? How can such different samples have their
averages turn out the way they did?
Part 3: Consider two jars. One jar contains 100 blue spheres, and the other jar contains 25 each of red,
blue, green, and yellow colors mixed together.
a.
If you were to remove 50 blue spheres from the jar containing just the blue spheres, what
would be total mass of spheres left in the jar? (Note that the masses of the spheres are given
in Part 2.)
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Chapter 2: Concept Explorations
b.
If you were to remove 50 spheres from the jar containing the mixture (assume you get a
representative distribution of colors), what would be the total mass of spheres left in the jar?
c.
In the case of the mixture of spheres, does the average mass of the spheres necessarily
represent the mass of an individual sphere in the sample?
d.
If you had 80.0 grams of spheres from the blue sample, how many spheres would you have?
e.
If you had 60.0 grams of spheres from the mixed-color sample, how many spheres would
you have? What assumption did you make about your sample when performing this
calculation?
Part 4: Consider a sample that consists of three green spheres and one blue sphere. The green mass is
3.00 g, and the blue mass is 1.00 g.
a.
Calculate the fractional abundance of each sphere in the sample.
b.
Use the fractional abundance to calculate the average mass of the spheres in this sample.
c.
How are the ideas developed in this Concept Exploration related to the atomic masses of the
elements?
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Chapter 2: Concept Explorations
2.26 Model of the Atom
Consider the following depictions of two atoms, which have been greatly enlarged so you can see the
subatomic particles.
a.
How many protons are present in atom A?
b.
What is the significance of the number of protons depicted in atom A or any atom?
c.
Can you identify the real element represented by the drawing of atom A? If so, what element
does it represent?
d.
What is the charge on element A? Explain how you arrived at your answer.
e.
Write the nuclide symbol of atom A.
f.
Write the atomic symbol and the atomic number of atom B.
g.
What is the mass number of atom B? How does this mass number compare with that of atom
A?
h.
What is the charge on atom B?
i.
Write the nuclide symbol of element B.
j.
Draw pictures like those above of 6 3 Li+ and 6 3 Li− atoms. What are the mass number and
atomic number of each of these atoms?
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Chapter 2: Concept Explorations
k.
Consider the two atoms depicted in this problem and the two that you just drew. What is the
total number of lithium isotopes depicted? How did you make your decision?
l.
Is the mass number of an isotope of an atom equal to the mass of the isotope of the atom? Be
sure to explain your answer.