Solutions
1
Solution:
Do not get hung up thinking “Hey, we never did one like this before!” Think in terms of
relationships.
To Find
You need
atoms
moles and
N
A
moles
mass and atomic mass
So the problem becomes more clear once you identify the relationships.
atoms
moles
N
A
atoms
grams
atomic mass
N
A
So now we know what we are looking for. We need the mass of the iron metal! Let’s see if
we can find that from the density. Density has units g/cm
3
. For iron the density is 7. 9
g/cm
3
. How can we use this to get grams? Let the units guide you. To get the mass of iron
metal it looks like we just need to multiply density by cm
3
which is a unit of volume.
mass g
7. 9 g/cm
3
sphere volume cm
3
7. 9 g/cm
3
4
3
4.0 in
2
2.54 cm/in
3
4338. g
Don’t forget the inch to cm conversion! Now that we have the mass we can find moles and
atoms. We have already written the formula we need.
atoms
grams
atomic mass
N
A
4338. g
55. 85 g/mol
6.022
10
23
atoms/mole
4. 678
10
25
atoms
2
Solution:
The thin sheet looks like a cylinder that is 1 atom high. The volume is
V
r
2
h
1. 1 cm
3
where h is the diameter of the atom. We are asked to find the diameter of the thin sheet.
d
2
2
h
1. 1 cm
3
The thickness of the thin metal sheet is equal to the
diameter
of the atom.
h
2. 78 Å

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