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
Amount of substance are atoms in of the isotope carbon-12. This number is called the Avogadro number and has the value It is the numerical value of the Avogadro constant, which has the unit mol, and relates the molar mass of an amount of substance to its mass. Therefore, the amount of substance of a sample is calculated as the sample mass divided by the molar mass of the substance. Amount of substance appears in thermodynamic relations such as the ideal gas law, and in stoichiometric relations between reacting molecules as in the law of multiple proportions. Another unit of amount of substance