This definition is useful because it allows chemists and scientists to easily convert between the number of moles of a substance and its mass in grams. For example, if you have a substance with a known number of moles, you can easily calculate its mass in grams by multiplying the number of moles by the molar mass of the substance (which can be found on the periodic table). Similarly, if you know the mass of a substance in grams, you can calculate the number of moles by dividing the mass by the molar mass. The fact that 1 mole of carbon atoms has a mass of exactly 12 grams also makes it easy to use carbon as a standard for comparing the masses of other elements or compounds.