Chemical formulas may be either “empirical” - as determined by experiments, or “molecular” - the true list of all elements and quantities. The empirical and molecular formulas have the same percent composition. The difference is that the molecular formula may be a multiple of the empirical one, based on molar quantities.
For example, a compound may have an empirical (basic ratio form) of
The percent composition – either by mass or by element – remains the same. That is the physical differentiation of the two. The common distinguishing factor is that a chemical formula will always be in the form of the expression, as shown, with the element symbol followed by the number of them in the compound.
A percentage will be a list of the elements separately, with their associated percentage defined either by mass or mole. In this example, the percentage composition is 64.9 percent carbon, 13.5 percent hydrogen, and 21.6 percent oxygen by mass.