Yes, formula mass is indeed applicable to ionic compounds.
Understanding Formula Mass
Formula mass is a term often used when discussing ionic compounds because these compounds don't exist as discrete molecules. Instead, they form a lattice structure of repeating ions. The formula mass, as opposed to molecular mass, is therefore the sum of the atomic masses of the atoms shown in the empirical formula of the compound.
Key Points:
- Definition: According to the provided reference, formula mass is "the sum of the atomic masses of constituent atoms in an ionic compound."
- Ionic Nature: It is primarily used for ionic compounds, which do not have discrete molecules, but ions as the constituent units.
- Lattice Structure: Ionic compounds form a crystal lattice where positive and negative ions are held together by electrostatic forces.
- Empirical Formula: The formula mass is calculated based on the empirical formula, which is the simplest whole-number ratio of atoms in the compound.
Why Use Formula Mass for Ionic Compounds?
Here's why we use formula mass rather than molecular mass for ionic compounds:
- No Discrete Molecules: Ionic compounds like sodium chloride (NaCl) don’t exist as individual NaCl molecules. Instead, they exist as a vast array of Na+ and Cl- ions in a continuous lattice structure.
- Ratio Representation: The formula, such as NaCl, represents the simplest ratio of the ions in the compound, not a single molecule.
- Calculating Compound Mass: To calculate the mass of a sample of an ionic compound, we sum the atomic masses from its empirical formula which gives us the formula mass. This allows us to determine the mass of a specific amount of the compound, even if it doesn't form molecules.
Calculation Example
For example, let's consider sodium chloride (NaCl):
- The atomic mass of Sodium (Na) is approximately 23 amu.
- The atomic mass of Chlorine (Cl) is approximately 35.5 amu.
- Therefore, the formula mass of NaCl is 23 + 35.5 = 58.5 amu.
This indicates that the mass represented by one formula unit of NaCl is 58.5 amu.
Summary Table
| Feature | Ionic Compounds | Molecular Compounds |
|---|---|---|
| Basic Unit | Ions in a lattice structure | Discrete molecules |
| Mass Calculation | Uses formula mass based on empirical formula | Uses molecular mass based on molecular formula |
| Example | NaCl (Sodium Chloride) | H₂O (Water) |
Conclusion
Formula mass is applicable and more appropriate than molecular mass for ionic compounds due to their lattice structure and lack of discrete molecules. The formula mass represents the sum of atomic masses in the empirical formula, helping calculate the mass of these compounds accurately.
