What is Formula Weight?

• A. The mass of one mole of a substance.
• B. The weight of a single atom.
• C. The sum of the atomic weights of everything that is in that particular formula.
• D. The weight of a sample.

Answer: (C)

Formula weight is the total mass contributed by all atoms in one formula unit of a compound. It is found by adding up the atomic weights of every atom in the chemical formula, counting each atom as many times as it appears. The result is typically given in atomic mass units per formula unit, and numerically it matches the molar mass when you use grams per mole. So, for a given formula, adding the atomic weights of everything inside that formula gives its formula weight.

This differs from the mass of a mole, which is the molar mass in grams per mole; it’s not the mass of a single atom, which would be just one atomic weight; and it’s not the weight of a sample, which depends on how much substance you have.