Answer:
The mole is a unit of measurement for the amount of substance or chemical amount. It is one of the base units in the International System of Units.
By this definition, a mole of any pure substance has a mass in grams exactly equal to that substance's molecular or atomic mass; e.g., 1mol of calcium-40 is approximately equal to 40g, because the Ca-40 isotope has a mass of 39.9625906 amu on the C-12 scale. In other words, the numerical value of a substance's molecular or atomic mass in atomic mass units is the same as that of its molar mass--the mass of one mole of that substance--in grams.