The thing to remember here is that in order to have
This number, which is known as Avogadro's constant, is essentially the definition of the mole. So
You can thus say that your sample of xenon difluoride will be equivalent to
#4.1 * 10^(19) color(red)(cancel(color(black)("molecules XeF"_2))) * "1 mole XeF"_2/(6.022 * 10^(23)color(red)(cancel(color(black)("molecules XeF"_2)))) = color(darkgreen)(ul(color(black)(6.8 * 10^(-5)color(white)(.)"moles XeF"_2#
The answer is rounded to two sig figs, the number of sig figs you have for the number of molecules of xenon difluoride.