Avogadro's number is defined as . When molecules of a compound are present, a mole of that compound is present. The quantity of a mole functions like the quantity of a dozen. A dozen atoms always means 12 atoms. Likewise, a mole of atoms always means atoms. A special feature of moles is that the mass in grams of one mole of a substance is equal to the mass in amu of one molecule (or atom) of the substance.
For example, the atomic mass of carbon-12 is 12. The means that a single carbon atom weighs 12 amu and a mole of carbon atoms weights 12 grams. This information can be used to convert between amu and grams.
Example:
How many moles of oxygen are present in 10g of
Since there is exactly one oxygen atom in each molecule of , this question is really asking how many moles of there are in 10g of .
