Relying on graduate department rankings for the evaluation of undergraduate programs can represent a counterproductive approach. For example, compare the student profile of Penn State (a school with an excellent graduate program in chemistry) to that of Swarthmore (a school with an excellent undergraduate program in chemistry), then draw your own conclusions:
https://nces.ed.gov/collegenavigator/?q=Pennsylvania+State&s=all&id=214777#admsns
https://nces.ed.gov/collegenavigator/?q=Swarthmore&s=all&id=216287#admsns