What is the Difference Between Formula Weight and Molecular Weight?

The difference between formula weight and molecular weight lies in the types of compounds they describe and the way they are calculated:

  • Formula Weight (Formula Mass):
  • Represents the sum of the atomic weights of the atoms in a compound's empirical formula.
  • Applicable to both ionic and molecular compounds.
  • For example, water (H₂O) has a formula weight of: $$2 \times (1.0079\; amu) + 1 \times (15.9994\; amu) = 18.01528\; amu$$.
  • Molecular Weight (Molecular Mass):
  • Represents the average mass of a molecule, calculated by adding together the atomic weights of the atoms in its molecular formula.
  • Applicable to molecular compounds only, as ionic compounds do not exist as discrete molecules.
  • For example, glucose, a molecular compound, has a molecular weight of: $$6 \times (12\; amu) + 12 \times (1.00794\; amu) + 6 \times (15.9994\; amu) = 180.0\; amu$$.

In summary, formula weight is the sum of the atomic weights of the atoms in a compound's empirical formula, while molecular weight is the average mass of a molecule calculated by adding together the atomic weights of the atoms in its molecular formula.

Comparative Table: Formula Weight vs Molecular Weight

The difference between formula weight and molecular weight lies in the type of substance being referred to. Here is a summary of the differences:

Formula Weight Molecular Weight
Represents the sum of the atomic weights of each atom in a compound's chemical formula. Represents the sum of the masses of all the atoms in the molecular formula of a molecule.
Used for compounds that do not exist as individual molecules, such as ionic compounds (e.g., salt, $$\ce{NaCl}$$). Used for compounds that exist as individual molecules, such as covalent compounds (e.g., water, $$\ce{H2O}$$).
Applicable to both ionic and covalent compounds. Applicable only to covalent compounds.

For example, table salt (NaCl) has a formula weight of 58.5 amu, which represents the sum of the atomic weights of its constituent elements, sodium (Na) and chlorine (Cl). In contrast, the molecular weight of a compound like water (H2O) represents the sum of the masses of all the atoms in its molecular formula, which is 18.01528 amu.