Question

How many moles of sulfur hexafluoride are present in $2.56E21$ molecules of this compound?

Answer 1

That many sulfur hexafluoride or $S{F}_6$ molecules are equivalent to $0.00425$ moles.
You know that $1$ mole of any compound contains $6.022\cdot {10}^{23}$ molecules of that compound - this is known as Avogadro's number. Since you have less that that number of molecules of sulfur hexafluoride to work with, you'll have less than $1$ mole.

So, the exact number of moles you have is
$2.56\cdot {10}^{21}$ molecules $\cdot \frac{1mole}{6.022\cdot {10}^{23}molecules}=0.4251\cdot {10}^{-2}=0.00425$ moles
The answer is rounded to three sig figs, the number of sig figs you have in $2.56\cdot {10}^{21}$.