What distinguishes molecular weight from formula weight?

• A. Molecular weight is the mass of one mole of a substance; formula weight is the sum of atomic weights in that formula.
• B. Molecular weight is the mass of one mole of a substance; formula weight is the mass of a single molecule.
• C. Molecular weight and formula weight are the same.
• D. Neither relates to weight.

Answer: (A)

Molecular weight describes the mass of one mole of molecules, calculated by adding up the atomic weights of all atoms in the molecule and expressing the result in grams per mole. Formula weight, sometimes called formula mass, is the mass corresponding to the formula unit—the sum of the atomic weights in the chemical formula that represents the smallest repeating unit of the substance. For true molecular compounds, those two values are the same number, because the molecule’s formula is the same as the substance’s formula. For ionic compounds, there isn’t a discrete molecule, so we use the formula unit, but the calculation is still the sum of the atomic weights in that formula. This is why the statement that molecular weight is the mass of a mole while formula weight is the sum of atomic weights in the formula best captures the distinction.