2. What is the difference between a formula unit for an ionic compound and a molecular formula

for a molecule?

In chemistry, a formula unit refers to the empirical formula of any ionic or covalent network solid compound used as an independent entity for stoichiometric calculations. A formula unit is thus the lowest whole number ratio of ions represented in an ionic compound.

While

A molecular formula comprises the chemical symbols for the constituent elements, followed by numeric subscripts specifying the number of atoms of each element present in a molecule.