What does molecular weight measure?

• A. The mass of a single molecule.
• B. The mass of one mole of a substance.
• C. The total weight of all atoms in the universe.
• D. The weight of the heaviest element.

Answer: (B)

The main idea is how much mass corresponds to the amount of substance you have. Molecular weight is used to describe how heavy a given amount of material is, specifically per amount of substance. In practice, the molecular weight (often called the molar mass) tells you the mass of one mole of those molecules, expressed in grams per mole. This ties the scale of atomic mass units for a single molecule to the mass per mole you use in calculations. So the best answer is that molecular weight measures the mass of one mole of a substance. The mass of a single molecule is a related idea (usually given in atomic mass units), but the concept described here centers on the amount of substance per mole.