1. Start with Avogadro's number:

Avogadro's number tells us that there are 6.022 x 10²³ particles (atoms, molecules, etc.) in one mole of a substance.

2. Find the moles of chlorine atoms:

* Divide the number of chlorine atoms by Avogadro's number:

(3.68 x 10²⁵ chlorine atoms) / (6.022 x 10²³ atoms/mole) = 6.11 moles of chlorine atoms

3. Relate moles of chlorine to moles of PCl3:

* The chemical formula PCl₃ tells us that there are 3 chlorine atoms for every 1 molecule of PCl₃.

* Therefore, there are 3 moles of chlorine atoms for every 1 mole of PCl₃.

4. Calculate moles of PCl3:

* Divide the moles of chlorine atoms by 3:

(6.11 moles Cl) / (3 moles Cl/mole PCl₃) = 2.04 moles of PCl3

Therefore, 3.68 x 10²⁵ chlorine atoms are contained in 2.04 moles of PCl3.