How many grams do 3.6 x10^20 atoms of silicon weigh?

First, we divide the number of atoms of silicon given in the problem above by Avogadro's number 6.6022 x10^23. This will give us the number of moles of silicon. Then, we multiply the number of moles by the molar mass of silicon. 
number of moles = (3.6 x 10^20 atoms/6.022x10^23 atoms) = 5.97x10^-4 moles
Then, multiplying by the molar mass
                  (5.97x10^-4 moles)(28.0855 g/mol) = 0.017 grams