Answer:
0.41678 g As
Explanation:
First convert from number of atoms to number of moles
3.35 x 10^21 atoms As x (1 mol As / 6.022x10^23 atoms As)
Cross out "atoms"
(3.35 x 10^21 / 6.022 x 10^23) mol As
Solve for number of moles
0.5563 x 10^-2 mol As
Convert number of moles of arsenic to grams of arsenic
0.5563 x 10^-2 mol As x (74.92 g As / 1 mol As)
Cross out "mol As"
0.5563 x 10^-2 x 74.92 g
Solve for grams
0.41678 g