Formula Weight and Molar Mass

The atomic weight of an element is the average mass of an atom of an element. The formula weight is the sum of the atomic weights of all of the ions in one formula unit of an ionic compound. The molecular weight is the sum of the atomic weights of all of the atoms in a molecule of a molecular compound. Both of these terms are somewhat old fashioned. A better term is molar mass, the mass in grams of one mole of the substance. This term is better for two reasons:

The 'weight' of an object varies with the strength of a gravitational field while the mass of a object reflects the amount of matter in that object. For nearly every chemical application, we need to know the amount of matter rather than how it responds to gravity.
The 'molar' mass of an ionic compound is the mass of a mole of a set of ions; the 'molar' mass of a molecular compound is the mass of a mole of molecules. The 'molar' mass of an element is the mass of a mole of atoms. A mole is a general term for a collection of Avogadro's Number of anything. By using a general term, we avoid labeling substances that do not contain molecules as molecular.

Remember that the subscripts in a formula indicate the number of ions in an ionic compound and the number of atoms in a molecule or polyatomic ion.

Concept Check: What is the formula weight of ammonium sulfate in amu? What is the molar mass of ammonium sulfate?

Answer: The formula of ammonium sulfate is (NH₄)₂SO₄.

FW = 2(14.0067 amu) + 8(1.00794 amu) + 32.065 amu + 4(15.9994 amu) = 132.140 amu

One mole of ammonium sulfate has a mass of 132.140 grams.

Note that the answer is rounded to the third decimal place due to application of the addition rule for significant figures.

This site was last updated 08/04/06