formula+weights

One important skill needed in chemistry calculations involves the determination of a molecule's mass in atomic mass units (or grams per mole). This value is referred to as the formula weight, or molecular mass, or molar mass.

A formula weight is simply the sum of the atomic masses of all elements in a molecule or compound.

To calculate formula weight, list all the elements, then list how many there are in the compound. For example, the formula for methane is CH4:

C 1 H 4

Then multiply the number of elements by that element's atomic mass from the periodic table:

C 1 x 12.01 H 4 x 1.008

which yields:

C 1 x 12.01 = 12.01 H 4 x 1.008 = 4.032

Finally, add up the results for the formula weight.

C 1 x 12.01 = 12.01 __H 4 x 1.008 = + 4.032__ = 16.042 amu

Try this link to an online formula weight calculator.

Download this worksheet on formula weights (1st page only):