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

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