For what type of compound are we unable to write a molecular formula?

Ionic compounds

Explanation:

Molecular formula consists of chemical symbols for constituent elements followed by the numeric subscripts describing number of the atoms of each element present in one molecule of the compound.

On the other hand, ionic compounds do not exist in the form of molecules. They are in the form of crystal lattice in the solid state which contains many ions each of cation and anion.

Thus, a ionic compounds are unable to write a molecular formula. Empirical formulas to used to represent them.