How many atoms are there in 1.45 x 10-17 mol of arsenic?

1 mole of any substance contains 6.02x10^23 "units" of that substance. This is Avogadro's constant.

Therefore to find the number of atoms you multiply the number of moles by this number.

=> 1.45x10^-17 x 6.02x10^23 = 8,730,000 atoms.