Formula Weight
Definition and meaning of Formula Weight in chemistry.
Formula weight is the sum of the atomic weights of all atoms in a chemical formula, expressed in atomic mass units (amu). It is calculated directly from the formula as written, without regard to whether the substance exists as discrete molecules.
In more detail
For molecular compounds, formula weight is numerically identical to molecular weight. The term "formula weight" is preferred for ionic and network compounds, such as NaCl or SiO2, which form extended lattices rather than discrete molecules, so no true "molecule" exists to weigh. To calculate it, multiply each element's atomic weight (from the periodic table) by its subscript in the formula, then sum the results. The formula weight in amu is numerically equal to the molar mass in g/mol, linking a single formula unit to a mole of that substance.
Key facts
| Field | General Chemistry |
|---|---|
| Example formula | NaCl |
| Units | amu (atomic mass units); equals g/mol for molar mass |
| Also called | Formula mass |
Sodium chloride, NaCl, has a formula weight of 22.99 (Na) + 35.45 (Cl) = 58.44 amu, corresponding to a molar mass of 58.44 g/mol.
Frequently asked questions
Is formula weight the same as molecular weight?
For molecular compounds they are numerically the same. "Formula weight" is used instead for ionic compounds like NaCl or CaCO3, which don't exist as individual molecules.
How do you calculate formula weight?
Multiply each element's atomic weight by its subscript in the chemical formula, then add all the values together.