What is defined as the mass of one mole of any substance?

The mass of one mole of any substance is known as molar mass. Molar mass is a crucial concept in chemistry, given that it allows scientists to convert between the mass of a substance and the number of moles present. The molar mass is expressed in grams per mole (g/mol) and is numerically equivalent to the average atomic or molecular mass of the substance expressed in unified atomic mass units (u).

For example, for a substance like water (H₂O), the molar mass can be calculated by adding together the molar masses of hydrogen and oxygen (approximately 1 g/mol for hydrogen and 16 g/mol for oxygen), resulting in a molar mass of about 18 g/mol for water. This aids in stoichiometric calculations, making it possible to relate masses of reactants and products in chemical reactions.

The other options refer to different concepts. Molecular weight typically refers to the average mass of a molecule, empirical mass refers to the mass of the simplest ratio of components in a compound, and atomic mass refers to the mass of an individual atom. These definitions highlight the distinction between them and emphasize the specificity of molar mass in the context of moles and substance quantification in chemistry.