What does the molar mass of a substance represent?

The molar mass of a substance represents the mass of one mole of that substance in grams. It is a fundamental concept in chemistry that allows scientists to relate the mass of a substance to the amount of substance at the molecular or atomic level. One mole of any substance contains Avogadro's number of entities, which is approximately (6.022 \times 10^{23}) molecules or formula units.

When the molar mass is expressed in grams per mole, it indicates how many grams of that substance correspond to one mole. For example, if the molar mass of water (H₂O) is about 18 grams per mole, this means that one mole of water has a mass of 18 grams. This relationship is crucial for stoichiometric calculations, which involve the conversion of mass to moles and vice versa in chemical reactions.

The other options do not accurately describe the concept of molar mass. The weight of one molecule would refer to a much smaller mass, the total number of atoms pertains to the molecular structure rather than mass, and density relates mass to volume rather than to moles. Thus, option B effectively captures the essence of what molar mass signifies in the context of chemistry.