Molar Mass and Moles

When the formula mass of an ionic compound is determined by the addition of its component atomic mass and expressed in grams, it is called the gram-formula mass or molar mass. An example is CaCO3

1Ca = 40

1C = 12

3O = 48 = (3 x 16)

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CaCO3 = 100 formula weight

100g = gram-formula weight or molar mass

The molar mass is equivalent to the mass of 1 mole of that material expressed in grams. The term molar mass or gram-molecular weight (gmw) can also be used when it is known that the material is a molecular substance and not and ionic lattice like NaCl or NaOH.

In some cases, there may be a question of the mass of 1 mole of an element if you are not told specifically what is referred to--the single atoms or a molecular state. An example of this situation may arise with hydrogen and other elements that form diatomic molecules. If you are asked the mass of 1 mole of hydrogen, you could say it is 1 gram if you are dealing with single atoms of hydrogen. This 1 mole of hydrogen would contain 6.02 x 10²³ molecules and since each molecule is composed of two atoms, 12.04 x 10²³ atoms.

The other elements that exist as diatomic molecules are oxygen, nitrogen, fluorine, chlorine, bromine, iodine, and astatine.