A formula unit is the empirical formula expressed in terms of the actual number of atoms present in one molecule of a compound. To determine the formula unit, use subscripts to show the relative numbers of each type of atom in the molecule. For example:

- Water (H2O): One molecule of water contains two hydrogen atoms and one oxygen atom, so its formula unit is H2O.

- Carbon dioxide (CO2): One molecule of carbon dioxide contains one carbon atom and two oxygen atoms, so its formula unit is CO2.

- Ammonia (NH3): One molecule of ammonia contains one nitrogen atom and three hydrogen atoms, so its formula unit is NH3.

- Calcium carbonate (CaCO3): One molecule of calcium carbonate contains one calcium atom, one carbon atom, and three oxygen atoms, so its formula unit is CaCO3.