The molecular formula for phtalic acid is is #C_8H_6O_4#. What is the empirical formula?
The empirical formula is the lowest ratio of kinds of atoms achievable while keeping them all integers. 8, 6, and 4 all have a common factor of two so we can divide this out, but there are no other common factors so we can't reduce it further.
If the compound was being studied experimentally the ratio of different kinds of atoms (so the empirical formula) is usually one of the easiest things to determine, and the molecular formula could be found later by a technique like mass spectrometry to scale up the empirical formula.