This text teaches you how to calculate the formula weight of a substance.
Before you begin, please take note that you should have a basic understanding of the following concepts:
- ions and molecules
- mole
- hydrated compounds
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Definition of Formula Weight
Formula weight is the sum of the atomic weights of all the atoms indicated in the chemical formula of a substance. It is the weight in grams of 1 mole of that substance, usually expressed as grams per mole. Hence, it is also sometimes called the molar weight.
The quantity of the atoms in a given formula is indicated by a numerical subscript after each atom. If no subscript is present after an atom, it is assumed to have a numerical subscript of one.
Formula Weight or Molecular Weight?
Formula weight is a generic term that may be used for both substances that exist as ions and molecules. Whereas, molecular weight is applicable only to substances that exist as molecules.
Uses of Formula Weight
Formula weight is used in the following:
- stoichiometry problems
- mole calculation
- percent composition calculation
- empirical formula calculation
Note: The following calculations are based on atomic weights with three significant digits.
Example Calculation 1
Formula weight of water, H2O :
H
O
2
1
x
x
1.008
15.999
=
=
2.016
15.999
18.015
Example Calculation 2
Formula weight of ammonium carbonate, (NH4)2CO3 :
N
H
C
O
2 x 1
2 x 4
1
3
x
x
x
x
14.007
1.008
12.011
15.999
=
=
=
=
28.014
8.064
12.011
47.997
96.086
Example Calculation 3
Formula weight of calcium chloride hexahydrate, CaCl2.6H2O :
Ca
Cl
H
O
1
2
6 x 2
6 x 1
x
x
x
x
40.078
35.453
1.008
15.999
=
=
=
=
40.078
70.906
12.096
95.994
219.074
