The molecular mass of a compound is the sum of the atomic masses of all the atoms in a molecule, while the formula mass is the sum of the atomic masses of all the ions in the formula unit of an ionic compound.
Here's a more detailed breakdown:
Molecular Mass
- Definition: Molecular mass is the mass of one molecule of a substance. It's calculated by adding the atomic masses of each element in the molecule, considering the number of atoms of each element as indicated by the molecular formula.
- Applicability: Molecular mass is primarily used for covalent compounds, which exist as discrete molecules.
- Units: Atomic mass units (amu) or Daltons (Da).
- Example: Water (H₂O)
- Atomic mass of Hydrogen (H) ≈ 1 amu
- Atomic mass of Oxygen (O) ≈ 16 amu
- Molecular mass of H₂O = (2 x 1) + 16 = 18 amu
Formula Mass
- Definition: Formula mass is the mass of one formula unit of a compound. It's calculated by adding the atomic masses of each element in the formula unit, considering the number of atoms of each element as indicated by the empirical formula.
- Applicability: Formula mass is primarily used for ionic compounds, which do not exist as discrete molecules but as a lattice of ions. It is also used for covalently bonded molecules where we are referring to a molar amount.
- Units: Atomic mass units (amu) or Daltons (Da).
- Example: Sodium Chloride (NaCl)
- Atomic mass of Sodium (Na) ≈ 23 amu
- Atomic mass of Chlorine (Cl) ≈ 35.5 amu
- Formula mass of NaCl = 23 + 35.5 = 58.5 amu
Differentiation: Molecular Mass vs. Formula Mass
The key difference lies in the type of compound to which the term is applied:
| Feature | Molecular Mass | Formula Mass |
|---|---|---|
| Compound Type | Primarily covalent compounds (discrete molecules) | Primarily ionic compounds (lattice structures) |
| Definition | Mass of one molecule. | Mass of one formula unit. |
| Structure | Discrete molecules | Extended lattice of ions |
| Example | Water (H₂O), Methane (CH₄) | Sodium Chloride (NaCl), Magnesium Oxide (MgO) |
In essence, while the calculation method is the same (summing atomic masses), the terms are used in different contexts based on the nature of the chemical bonding within the compound. Molecular mass refers to molecules, whereas formula mass refers to formula units typically found in ionic compounds or to a molar amount of a covalently bonded molecule.
