The formula mass of a substance represents the sum of the atomic masses of all the atoms present in the substance's formula unit.
Understanding Formula Mass
Formula mass is primarily used for ionic compounds and other substances that do not exist as discrete molecules. Instead, they exist as a lattice of ions. In these cases, the term "molecular mass" isn't strictly applicable because there aren't distinct molecules. Therefore, we use "formula mass" to refer to the mass of the formula unit representing the compound's composition.
Calculating Formula Mass
To calculate the formula mass:
- Identify the formula of the substance.
- Look up the atomic masses of each element in the formula from the periodic table.
- Multiply each element's atomic mass by the number of times that element appears in the formula.
- Sum up the results from step 3. The result is the formula mass, typically expressed in atomic mass units (amu).
Example: Sodium Chloride (NaCl)
Let's calculate the formula mass of sodium chloride (NaCl):
- Formula: NaCl
- Atomic mass of Sodium (Na): approximately 22.99 amu
- Atomic mass of Chlorine (Cl): approximately 35.45 amu
- Calculation: (1 x 22.99 amu) + (1 x 35.45 amu) = 58.44 amu
Therefore, the formula mass of NaCl is approximately 58.44 amu.
Formula Mass vs. Molecular Mass
While often used interchangeably in introductory contexts, there's a subtle distinction:
- Formula Mass: Used for ionic compounds and substances that do not exist as discrete molecules.
- Molecular Mass: Strictly used for covalent compounds that exist as discrete molecules. It represents the sum of atomic masses in a molecule.
Essentially, molecular mass is a specific type of formula mass. If the substance is molecular, using "molecular mass" is generally preferred, while "formula mass" is more general and can be applied to both ionic and molecular substances.
