The calculation of empirical formulas will work up to about Mw=250.
The derived formula contains no other elements than those entered above. Often only carbon and hydrogen are determined. If there is no further information the remainder can be assumed to be oxygen. In the case of a non-integral number of unsaturations (Ω) you may want to use twice the empirical formula.
It is also possible that you are lacking a halogen or a counter ion.