To find the number of atom we multiply the
No.of moles number of mole with Avogadro's number.
As 1mole of arsenic contains contains 6.02x10^23 unit of substance which is a constant value e for 1mooe
So;
1.45x10^-17moles contain=6.02x10^23 multiply with 1.45x10^-17
= 8,730,000 atoms of arsenic are present in above mentioned moles of arsenic