Chemistry Reference
In-Depth Information
Example 5
How many molecules does 3.00 moles of carbon dioxide represent?
# of molecules = 3.00 moles ×
6.02 × 10
23
molecule
mole
= 1.81 × 10
23
molecules of CO
2
Other than the fact that the answer to Example 5 is large, and in scien-
tific notation, it is just as simple as Example 4!
Let's suppose the question from Example 5 was worded somewhat dif-
ferently. How would you find the number of atoms in 3.0 moles of carbon
dioxide? To answer this question, you must understand what it is asking.
This question is asking for “parts of the whole,” the “whole” being mol-
ecules and the “parts” being atoms. Each molecule of carbon dioxide (CO
2
)
is made up of three atoms, 1 carbon and 2 oxygen. You would find out the
number of molecules, and then multiply by 3 atoms/molecule to get your
final answer. This question can be likened to asking, “How many tires (not
counting spares) are there in 3 dozen cars?” Let's answer both of these
questions.
Example 6
How many tires (not counting spares) are there in 3 dozen cars?
12
1 doz
4 tires
car
# of tires = 3.0 doz car ×
×
= 144 tires
Example 7
How many atoms does 3.0 moles of carbon dioxide represent?
# of atoms = 3.0 moles ×
6.02 × 10
23
molecules
mole
3 atoms
molecule
×
= 5.4 × 10
24
atoms
Changing the Number of Particles to the Number of Moles
Suppose a particular chemical reaction results in the production of
1.5 × 10
23
molecules of hydrogen (H
2
) and you want to determine the num-
ber of moles this represents. The solution to this problem may seem obvi-
ous to you, but if not, think again about a dozen. If you had six doughnuts,
how many dozen doughnuts would you have? Dividing 6 by 12 you would
find that you have 0.5 (or 1/2) a dozen doughnuts. We would solve the
mole problem the same way!
