Chemistry Reference
In-Depth Information
Molecular and Molar Masses
Compound Name and
Molecular Formula
Molecular Mass (mass
of one molecule)
Molar Mass (mass of
6.02 × 10
23
molecules)
Water (H
2
O)
Carbon Dioxide (CO
2
)
Glucose (C
6
H
12
O
6
)
18.0 u
18.0 g
44.0 u
44.0 g
180 u
180 g
Now you know what the mole is and why it is so important to chemistry.
If you find yourself getting confused in the future, just think about the
“dozen.” If you don't understand what to do when someone asks you to
“find the mass of 2.0 moles of carbon dioxide molecules,” just substitute
the word dozen for mole. If someone asked you to find the mass of 2.0
dozen carbon dioxide molecules, you would simply determine the mass of
one molecule of carbon dioxide and multiply by 24 (2 dozen = 2 × 12 = 24).
If you need to find the mass of 2.0 moles of carbon dioxide molecules, you
could simply determine the mass of one molecule of carbon dioxide and
multiply by 2 × Avogadro's number (2 × (6.02 × 10
23
)). Look at these two
solutions shown with the factor-label method here:
Example 1
Determine the mass of 2 dozen carbon dioxide molecules.
44.0 u
molecule
12 molecule
doz
2 doz
×
×
= 1056 u = 1060 u
That should seem easy enough. Now, apply the same technique to find-
ing the mass of 2 moles of carbon atoms.
Example 2
Determine the mass of 2 moles of carbon dioxide molecules.
44.0 u
molecule
6.02 × 10
23
molecule
mole
2 mole
×
×
= 5.30 × 10
25
u
This number is so large that you can see why it is preferable to change
atomic mass units to grams when dealing with molar mass. Example 3 will
show the same calculation with an added conversion to change atomic mass
units to grams.
