Chapter 5 · Molecules and Compounds
5.11Formula Mass: The Mass of a Molecule or Formula Unit
12 min · two checks
Predict
What is the formula mass of H₂O, using H = 1.008 u and O = 16.00 u?
The idea
- Calculate formula mass from a formula and atomic masses.
- Treat ionic formula units and molecules with the same arithmetic.
Formula mass is the sum of atomic masses in one formula as written. For a molecule it is also called the molecular mass. For an ionic compound there is no molecule, so you total one formula unit: NaCl is 22.99 u + 35.45 u = 58.44 u. Multiply any subscript, including those created by parentheses. Ca(NO₃)₂ includes two nitrogens and six oxygens.
Use the atomic masses supplied by the problem or a consistent table, and do not round each atomic mass to an integer unless you are estimating. The arithmetic is addition, so the decimal place is limited by the least precise mass you added, but textbook values are usually precise enough that the sum keeps two digits after the decimal. This number, with the unit g/mol instead of u, is the molar mass you will use constantly in the next chapter. The numerical value does not change.
Keep these
- Formula mass = Σ (subscript × atomic mass).
- Parentheses multiply every atom inside.
- The same number in g/mol is the molar mass.
Worked path
Calculate the formula mass of Ca(NO₃)₂. Ca 40.08, N 14.01, O 16.00.
Observe
One Ca, two N, six O.