Gram Formula Weight
Definition and meaning of Gram Formula Weight in chemistry.
Gram formula weight is the mass, in grams, of one mole of a substance's formula units, numerically equal to its formula weight (the sum of atomic weights in the empirical formula). The term is used especially for ionic and network compounds, which do not exist as discrete molecules.
In more detail
Because ionic compounds like NaCl form extended crystal lattices rather than individual molecules, chemists describe them using a formula unit instead of a molecule. Adding the atomic weights in that formula unit gives the formula weight (in amu); expressing that same number in grams gives the gram formula weight, which equals the mass of 6.022 x 10^23 formula units (one mole). This quantity is numerically identical to molar mass and lets chemists convert between grams and moles for stoichiometry calculations involving ionic compounds.
Key facts
| Formula (example) | NaCl |
|---|---|
| Field | General Chemistry |
| Units | grams per mole (g/mol) |
| Equivalent quantity | molar mass |
Sodium chloride, NaCl, has a formula weight of 58.44 (22.99 for Na plus 35.45 for Cl), so its gram formula weight is 58.44 g. This mass contains one mole of Na+ ions and one mole of Cl- ions.
Frequently asked questions
How does gram formula weight differ from gram molecular weight?
Gram formula weight applies to substances without discrete molecules, such as ionic compounds and network solids, and is based on the empirical formula unit. Gram molecular weight applies to true molecular substances and is based on the actual molecular formula.
Is gram formula weight the same as molar mass?
Yes, numerically. Gram formula weight is the mass of one mole of formula units expressed in grams, which is exactly what molar mass (g/mol) represents.