Methoxychlor , a garden insecticide . What is the total no of moles of atoms in 3mg sample ?
 

Dear student , 
The structure of methoxychlor is as follows
(M.wt=345..644 g)

M.wt                  =           1 mole
345.644 g           =         1mole
3mg or 3$\times$10^-3g=         $\frac{1}{345.644}\times 3\times {10}^{-3}$ = $8.647\times {10}^{-6 }moles$
Number of atoms in 8.647$\times {10}^{-6}$ moles  = $6.023\times {10}^{23}\times 8.67\times {10}^{-6}$ = 5.22$\times$10¹⁸ atoms
Regards!