Chapter 6 · Chemical Composition
6.9Calculating Molecular Formulas for Compounds
11 min · two checks
Predict
The empirical formula is CH₂O and the molar mass is 180 g/mol. What is the molecular formula?
The idea
- Find the integer multiple that connects empirical and molecular formulas.
- Require a molar mass; composition alone is not enough.
Compute the empirical formula mass, then n = (molar mass) / (empirical formula mass). The n you want is a whole number. Multiply each empirical subscript by n. If n comes out 2.01, it is 2. If it comes out 1, the empirical and molecular formulas are the same, which is true of water, carbon dioxide, and many small molecules.
Molar mass might be given directly, or it might come from a mass-spectrometer-style problem, or later from the ideal gas law. The formula step itself does not care where the molar mass came from. It does care that you use the empirical mass, not one atom’s mass, in the division.
Keep these
- n = molar mass / empirical formula mass.
- Multiply all empirical subscripts by the same n.
- n = 1 is allowed. Not every molecule is a multiple.
Worked path
A compound is 85.6% C and 14.4% H, with molar mass 56.1 g/mol. Find the molecular formula.
Observe
100 g gives 85.6 g C and 14.4 g H. Moles: 7.13 mol C and 14.3 mol H.