The
empirical formula gives the
simplest whole number ratio of the atoms in a substance.
An example:
Hydrogen peroxide has the empirical formula HO with 1 H and 1 O.
Ethanoic acid has the empirical formula CH2O with 1 C 2 H and 1 O.
In the actual formula for a substance the
number of each type of atom that is present
may be a multiple of that indicated by the empirical formula.
Hydrogen peroxide may exist as H2O2 or H3O3.
The actual
molar mass of a substance can be determined by titration or using mass spectrometry.
If the experimentally-determined molar mass of the molecule is
different from the
total mass of atoms in the empirical formula, the empirical formula is NOT the actual formula of the substance.
For example hydrogen peroxide has the empirical formula HO.
For OH, the molar mass is 17 g mol–1
The experimental molar mass of hydrogen peroxide is 34 g mol–1
The
actual formula for the substance can be determined if both the molar mass and the empirical formula are known.
- Divide the experimental molar mass by the molar mass of the empirical formula to obtain the number by which all subscripts in the empirical formula must be multipled.
For hydrogen peroxide, this number is 2 (34 g mol–1 divided by 17 g mol–1).
- Multiply all atoms in the empirical formula by this factor to deduce the molecular formula.
For hydrogen peroxide this factor is 2, and H1 × 2O1 × 2 gives H2O2.
The actual structure of H2O2 is HO–OH.