How many moles of pcl3 contain 3.68 × 1025 chlorine atoms?

To answer the question above, we take note first that every mole of PCl₃ contains 3 atoms of Cl. First, we divide the given number by 3.
                                   3.68 x 10²⁵ / 3 = 1.227 x 10²⁵ 
Then, we divide the answer by the Avogadro's number which is equal to 6.022 x 10²³. That is,
                                    1.227 x 10²⁵ / (6.022 x 10²³) = 20.37
Thus, there are 20.37 mol of PCl₃.