What is the Difference Between Formula Weight and Molecular Weight?
🆚 Go to Comparative Table 🆚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.
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