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.