A tentative empirical formula works out as ClO3.5. What is the correct empirical formula?

1. Formula subscripts must be whole numbers, so a subscript of 3.5 cannot stand. 2. Dividing by the smallest subscript has already been done, so the fix is to scale both subscripts up. 3. The decimal .5 is cleared by multiplying every subscript by 2. 4. That turns Cl1 into Cl2 and O3.5 into O7. 5. The empirical formula is therefore Cl2O7. 6. Rounding 3.5 up to 4 to give ClO4 changes the atom ratio and is the main trap here. 7. Multiplying only the oxygen subscript would break the ratio too, since both subscripts must be scaled by the same factor. _Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.2 Determining Empirical and Molecular Formulas_