Background: Amount of substance is a fundamental physical quantity which has mole (mol) as it's SI unit. Therefore all expressions for amount of substance should have the unit mole on simplification. Therefore, if A = Amount of substance (normally in moles), m = Mass of the substance in a particular unit (normally in grams), M is the mass per unit amount of substance (normally in grams/mole), then, A = m/M If the substance is a molecule then M is known as molar mass of the substance. But in my textbook and in many websites on the internet, I have encountered the phrase number of moles. I think it refers to the amount of substance or may be the numerical part in the amount of substance. The formulas given there were strange. If n is the number of moles, m is the mass of the substance in grams and x is the atomic weight (for atoms of elements) or molecular weight (for molecules of elements and compounds), then, n = m/x This equation is not dimensionally correct if I am right. My questions: First of all i would like to ask whether amount of substance and number of moles refer to the same thing or is it that amount of substance has a unit along with a numerical value whereas number of moles does not have a unit and represents the numerical value in the magnitude of the amount of substance. Secondly, between equations 1 and 2, which one is completely correct (both in meaning and dimension)? Thirdly, since in some place I have encountered gram atomic/molecular mass in place of molar mass, i would like to know what are the differences between both and do they have the same units or different units?