The International Committee for Weights and Measures defines one mole as the number of atoms in exactly 12 grams of carbon-12 (Figure 2). Experiments counting the number of carbon-12 atoms in a 12-gram sample have determined that this number is 6.02214076 x 10²³. Regardless of whether the substance is carbon-12 or doughnuts, one mole represents the same number of each of these things.

Figure 2: Carbon-12

Scientists have then defined the molar mass of a substance as the mass of 6.02214076 x 10²³ units of that substance. So, the molar mass of yeast-raised doughnuts is 301,000,000,000,000,000,000,000,000 grams. With doughnuts, this is not very useful. However, it is quite useful if we apply it to other substances, especially elements. By standardizing the number of atoms in a sample of an element, we also get a standardized mass for that element that can be used to compare different elements and compounds to one another. Carbon's molar mass is 12.01 grams, which represents the combined mass of 6.02 x 10²³ carbon atoms. However, other elements have different molar masses; for example, 6.02 x 10²³ sulfur atoms have a mass together of 32.06 grams, which is sulfur's molar mass.

Along with telling us the mass of one mole of an element, molar mass also acts as a conversion factor between the mass of a sample and the number moles in that sample. For example, 24 grams of carbon atoms would be equal to two moles since 24 grams divided by the mass of one mole (12.01) equals 2. Further, Avogadro’s number acts as the conversion factor for converting between the number of moles in a sample and the actual number of atoms or molecules in that sample. Extending our example, two moles of carbon atoms contains 2 times 6.02 x 10²³ atoms, which equals 12.04 x 10²³ atoms, which can be written as 1.204 x 10²⁴ atoms.