The empirical formula gives the simplest whole number ratio of the atoms in a substance.
In the actual formula for a molecular substance the number of each type of atom that is present may be a multiple of that indicated by the empirical formula.
The factor by which all of the subscripts in the empirical formula must be multiplied to get the molecular formula can be determined if the relative mass of the molecular substance is known. This can be determined by titration or using mass spectrometry.
If the experimentally-determined relative mass of the molecule is different from the total mass of atoms in the empirical formula, the empirical formula and the molecular formula are not the same.
hydrogen peroxide
For example hydrogen peroxide has the empirical formula HO.
For OH, the total relative mass 17 (16 for O and 1 for H).
The experimentally-determined relative mass of hydrogen peroxide molecules is 34.
The actual formula for the substance can be determined if both the relative mass of the molecule and the empirical formula are known.
In the actual formula for a molecular substance the number of each type of atom that is present may be a multiple of that indicated by the empirical formula.
The factor by which all of the subscripts in the empirical formula must be multiplied to get the molecular formula can be determined if the relative mass of the molecular substance is known. This can be determined by titration or using mass spectrometry.
If the experimentally-determined relative mass of the molecule is different from the total mass of atoms in the empirical formula, the empirical formula and the molecular formula are not the same.
hydrogen peroxide
For example hydrogen peroxide has the empirical formula HO.
For OH, the total relative mass 17 (16 for O and 1 for H).
The experimentally-determined relative mass of hydrogen peroxide molecules is 34.
The actual formula for the substance can be determined if both the relative mass of the molecule and the empirical formula are known.
- Divide the relative mass of the molecule by the total relative mass of the empirical formula to obtain the number by which all subscripts in the empirical formula must be multipled.
- Multiplying all atoms in the empirical formula gives the molecular formula.
For hydrogen peroxide this factor is 2, and the molecular formula is H2O2.
The actual structure of H2O2 is HO–OH.
The actual structure of H2O2 is HO–OH.