While the calculation process for formula mass and molecular mass is identical, the terms are used differently due to the distinct nature of the substances they describe. This distinction arises from the types of chemical bonds holding the atoms together.
Understanding the Difference
The key difference lies in whether the compound is covalent or ionic:
- Molecular Mass: This term is specifically used for covalent compounds. These compounds consist of molecules where atoms are held together by shared electrons. Covalent compounds are formed between two or more non-metal atoms. For instance, water (H₂O) and carbon dioxide (CO₂) are examples of covalent compounds, and their masses are referred to as molecular mass.
- Formula Mass: This term is used for ionic compounds. These compounds form a lattice structure of positively and negatively charged ions held together by electrostatic forces. Ionic compounds consist of metal and non-metal atoms. The mass of ionic compounds, such as sodium chloride (NaCl), is described using the term formula mass, since ionic compounds do not exist as single, discrete molecules.
Why the Different Terminology?
The distinction is rooted in the fundamental difference in how these substances exist:
- Molecules are discrete units of covalently bonded atoms.
- Ionic compounds are not made of molecules but rather large 3-dimensional crystal structures where ions are attracted to each other. The smallest ratio of ions in an ionic compound is defined by its formula.
Calculation Process:
Regardless of whether we are calculating molecular mass or formula mass, the procedure remains the same:
- Identify the atoms: Determine the types and number of atoms present in the molecule or the formula unit.
- Obtain Atomic Masses: Find the atomic masses of each element involved from the periodic table.
- Multiply and Sum: Multiply the atomic mass of each element by the number of atoms of that element and then add all the resulting masses together.
Summary Table
| Feature | Molecular Mass | Formula Mass |
|---|---|---|
| Applies to | Covalent Compounds | Ionic Compounds |
| Type of bond | Covalent (sharing of electrons) | Ionic (electrostatic attraction) |
| Structure | Discrete molecules | 3D crystal lattices |
| Composition | Non-metals only | Metals and non-metals |
Practical Example
To better visualize, consider the following examples:
-
Carbon Dioxide (CO₂):
- One carbon atom with a mass of approximately 12.01 amu.
- Two oxygen atoms with a mass of approximately 16.00 amu each.
- Molecular mass = (1 × 12.01) + (2 × 16.00) = 44.01 amu
-
Sodium Chloride (NaCl):
- One sodium ion with a mass of approximately 22.99 amu.
- One chloride ion with a mass of approximately 35.45 amu.
- Formula mass = 22.99 + 35.45 = 58.44 amu
In conclusion, although the method of calculation is the same, the terminology—molecular mass versus formula mass—reflects the fundamentally different bonding and structure characteristics of covalent and ionic substances.
