1. Avogadro's Number:

* Avogadro's number tells us the number of atoms or molecules in one mole of a substance. It's approximately 6.022 x 10^23.

2. Calculation:

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

(1.20 x 10^25 atoms) / (6.022 x 10^23 atoms/mol) ≈ 19.9 moles

Therefore, 1.20 x 10^25 atoms of phosphorus represent approximately 19.9 moles of phosphorus.