As you might expect, the term molecular
mass refers to the mass of a molecule. Fortunately, calculating the
molecular mass is no harder than understanding the concept. The masses shown on the
periodic table are the atomic masses. To determine the mass of a
molecule, simply add up all of the atomic masses for the atoms that make up the
molecule. As a general rule, whenever you take mass information of the periodic
table, round to three significant digits. Here are two examples of molecular mass
calculations.
Table
5-6a. Determining the Molecular Mass |
Molecular Mass of CO2 |
Molecular Mass of C6H12O6 |
| C = 12.0 u x 1 atom = 12.0 u O = 16.0 u x 2 atoms = 32.0 u
--------
Total
=
44.0
u |
C = 12.0 u x 6 atoms =
72.0 u H = 1.01 u x 12 atoms =
12.1 u
O = 16.0 u x 6 atoms = 96.0 u
--------
Total =
180.1 u |
As you can see, to find the molecular
mass of carbon dioxide we simply look up the atomic masses of carbon and oxygen.
Multiply oxygen's atomic mass by two because there are two oxygen atoms in each molecule
of carbon dioxide. Then add the masses together. The units are atomic
mass units with the symbol (u).
Finding the formula mass
of an ionic compound is just as easy, in fact, the calculation is exactly the same.
The only difference has to do with terminology. We don't call ionic compounds
molecules, so you can't find the molecular mass of an ionic compound. Instead, we do
the exact same calculation, but we call the results the formula mass of
the compound. The table below shows how to calculate the formula mass of two ionic
compounds.
Table
5-6b, Determining the Formula Mass |
Formula Mass of CuSO4 |
Formula Mass of Ca(OH)2 |
| Cu = 63.5 u x 1 atom = 63.5 u S = 32.1 u x 1 atom = 32.1 u
O = 16.0 u x 4 atoms = 64.0 u
-------
Total =
159.6
u |
Ca = 40.1 u x 1 atom = 40.1 u O = 16.0 u x 2 atoms = 32.0 u
H = 1.01 u x 2 atoms = 2.02 u
--------
Total =
74.1 u |
Now, use the links below to practice and test what you have
learned.
Molecular Mass and
Formula Mass Quizzes |
| |
Please forward all questions, comments and criticisms to Gregory L. Curran.
© Copyright 2004 Fordham Preparatory School, All Rights Reserved.
Last Modified February 07, 2008 |
