Chemistry Reference
In-Depth Information
Whichever type of compound you are dealing with, calculating the mass
of a compound is very simple. All you need to do is add up the masses of
each atom involved in the compound. Look up each of the atomic masses
and multiply them by the subscripts associated with each element. Add
these totals together to get the overall mass.
First, we will do an example involving the molecular mass of water.
Example 1
Find the molecular mass of water, H
2
0.
H
2
= 1.01 u/atom x 2 atoms = 2.02 u
O = 16.0 u/atom x 1 atom =+ 16.0 u
Total =
18.0 u
Now, let's see an example involving the formula mass of carbon dioxide.
Example 2
Find the formula mass of carbon dioxide, CO
2
.
C = 12.0 u/atom x 1 atom = 12.0 u
O
2
= 16.0 u/atom x 2 atoms =+ 32.0 u
Total =
44.0 u
As you can see, whether you are dealing with a molecular compound
or an ionic compound, the method for solving the problem is identical. The
key is to be sure that you take all of the atoms in the formula into account.
Students are sometimes confused by parentheses, so let's try a couple
of examples that include them.
Example 3
Find the formula mass of carbon dioxide, Ca(OH)
2
.
Ca = 40.1 u/atom x 1 atom = 40.1 u
O
2
= 16.0 u/atom x 2 atoms = 32.0 u
H
2
= 1.01 u/atom x 2 atoms =+ 2.02 u
Total =
74.1 u
