Empirical Formula (OCR GCSE Chemistry A (Gateway)): Revision Note

Empirical Formula

• Empirical formula is the simplest whole number ratio of atoms of each element in a molecule 

• The molecular formula tells you the actual number of atoms of each element in a molecule

• From the molecular formula, we can deduce the empirical formula 

• The molecular formula will either be given to you or can be worked out from a picture 

• Sometimes, the empirical formula and molecular formula are the same e.g. CO₂

Empirical Formula of Ionic Compounds

• If it is a dot-and-cross diagram, then just count the number of atoms of each element

• This is then equal to the empirical formula of the compound

• If it is a 3D lattice structure, identify the ions in the lattice

• Write them down and balance the charges so that the overall charge is zero 

• The formula of the compound is the empirical formula 

Percentage Composition

• The percentage by mass of an element in a compound can be calculated using the following equation:

$\%\mathrm{mass}\mathrm{of}\mathrm{an}\mathrm{element}=\frac{A_{\mathrm{r}}\times \mathrm{number}\mathrm{of}\mathrm{atoms}\mathrm{of}\mathrm{the}\mathrm{element}}{M_{\mathrm{r}}\mathrm{of}\mathrm{the}\mathrm{compound}}\times 100$