Formula mass is the sum of the atomic masses of each atom in a chemical formula. It's used for ionic and covalent compounds alike and is expressed in atomic mass units (amu).
Understanding Formula Mass
Formula mass represents the mass of one formula unit of a compound. This is particularly relevant for ionic compounds, where discrete molecules don't exist. Unlike molecular mass, which specifically refers to the mass of a molecule, formula mass is a more general term. To calculate the formula mass, you simply add up the atomic masses of all the atoms in the chemical formula, taking into account the number of atoms of each element present. You can find the atomic masses on the periodic table.
Example: Sodium Chloride (NaCl)
Consider sodium chloride (NaCl), common table salt. It's an ionic compound composed of sodium ions (Na+) and chloride ions (Cl-) in a 1:1 ratio.
To calculate the formula mass of NaCl:
- Identify the elements: Sodium (Na) and Chlorine (Cl).
- Find their atomic masses:
- Na: Approximately 22.99 amu
- Cl: Approximately 35.45 amu
- Add the atomic masses: 22.99 amu + 35.45 amu = 58.44 amu
Therefore, the formula mass of NaCl is 58.44 amu.
In summary, formula mass is the sum of atomic masses in a formula unit, like NaCl, and is expressed in amu.
