Find empirical formula when individual masses are known
An empirical formula is, according to IUPAC’s Gold Book definition:


  Formed by juxtaposition of the atomic symbols with their appropriate subscripts to give the simplest possible formula expressing the composition of a compound


Wikipedia goes even further:


  the simplest positive integer ratio of atoms of each element present in a compound


and then


  An empirical formula makes no reference to isomerism, structure, or absolute number of atoms.


So, NiF2 is an empirical formula, while Ni2F4 simply isn't. Another example is given by, again, Wikipedia:


  For example, formaldehyde, acetic acid and glucose have the same empirical formula, CH2O


Their molecular formulas (which account for the absolute number of atoms in a molecule) differ, though: formaldehyde CH2O, acetic acid C2H4O2, glucose C6H12O6.