Answer: mole
A formula unit represents the simplest ratio of elements in a mole of an ionic compound.
Step-by-step explanation:
The formula unit, also known as the empirical formula, gives the simplest whole number ratio of the components of the elements in a mole of an ionic compound.
Thus, the formula unit and the molecular formula of a compound can be the same.
For example: Glucose with C6H12O6 will have a formula unit of CH2O. Thus, the simplest ratio of carbon, hydrogen and oxygen in glucose is 1:2:1