A balloon containing 0.0400 mil of gas with a volume of 500mL was expanded to 1.00L what is the final number of mols?
Well, clearly the final number of moles is the same as the initial number of moles.
Mass is conserved in every chemical process, and (most) every physical process. Here, mass and moles, number of molecules are certainly conserved, because all we have done is expand the volume. So the final number of molecules, of moles, is the same as the first.