Molar mass Molar mass In chemistry, the molar mass "M" is a physical property defined as the mass of a given substance (chemical element or chemical compound) divided by the amount of substance. The base SI unit for molar mass is kg/mol. However, for historical reasons, molar masses are almost always expressed in g/mol. In simple terms, molar mass of a substance is the total weight of that substance (in either kilogram or gram) for one mole of that substance. That is, the weight of a substance for 6.02214076×10^23 molecules or atoms of that substance. As an example, the molar mass of
Barn (unit) of a nucleus with an electric field gradient. While the barn is not an SI unit, the SI standards body acknowledges its existence due to its continued use in particle physics. The etymology of the unit barn is whimsical: during Manhattan Project research on the atomic bomb during World War II, American physicists at Purdue University needed a secretive unit to describe the approximate cross sectional area presented by the typical nucleus (10 m) and decided on "barn". This was particularly applicable because they considered this a large target for particle accelerators that needed to have direct strikes on nuclei