What does molecular weight represent?

Molecular weight specifically refers to the sum of the atomic weights of all the atoms that make up a molecule. Each element has a defined atomic weight based on its atomic mass, which is usually expressed in atomic mass units (amu). When calculating the molecular weight of a compound, the atomic weights of the constituent atoms are combined according to their respective quantities in the molecule.

For example, in a water molecule (H₂O), the molecular weight is calculated by adding the atomic weights of the two hydrogen atoms (approximately 1 amu each) and one oxygen atom (approximately 16 amu), resulting in a molecular weight of about 18 amu. This concept is critical in chemistry for understanding stoichiometry, reactions, and various properties of substances.

The other choices do not accurately represent what molecular weight is. The weight of a single atom pertains to atomic weight, the total weight of substances during a reaction relates to the law of conservation of mass, and the ratio of mass to volume describes density, which is unrelated to molecular weight.