What is the empirical formula of #C_4H_8O_2#?
Now I was able to do that even without a calculator and without a second thought. Why?
I will one day get sick of saying this, but the empirical formula is the simplest whole number ratio that defines constituent atoms in a species. The molecular formula is ALWAYS a multiple of the empirical formula. Of course, the multiple MAY be ONE.
The empirical formula is what we find by experiment. Normally we have to take the empirical formula and compare it to a measured molecular mass in order to calculate the multiple, and thus the molecular formula. We have the cart before the horse here, but assignment of molecular formula is straightforward.