How many moles are 1.20 x10^25 atoms of phosphorus?

1 mole of any substance contain Avogadro's number of molecules so we can calculate the number of moles by dividing the provided number of atoms over Avogadro's number to obtain the number of moles 

Answer : 19.9 moles of phosphorus

  Solution : Atoms / Avogadro's number (6.022 x 10²³ atoms/mol)

 1.20 x 10²⁵ atoms / 6.022 x 10²³ = 19.927

The answer is c) pyrite