"Background radiation" is the overall radiation in the general area. If, for example, you want to use a geiger counter to determine the radiation from a Radium sample, you would first stand well away from the sample and set the sensitivity of the geiger counter so that it will not register the "background radiation".
It might well be that Carbon-14 and Potassium-40 are the greatest overall contributors to background radiation but surely are not "all there is"- if you standing in a Uranium mine, you might find that Uranium-235 contributed to the background radiation!
As far as the "empirical" and "molecular" formulas are concerned, I think you mean by "molecular formula", the formula based on actually knowing how many atoms of each element are in the molecule. The "empirical" formula is based on measurements and, so, is only approximate. However, since the number of molecules HAS TO BE AN INTEGER, if the empirical formula say "2.00003 atoms of oxygen" you can make a pretty good guess as to what the molecular formula is!
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