Answer:
19.9 atoms
Step-by-step explanation:
Grams --- Moles --- Atoms
You're converting from atoms (molecules) to moles.
You do not have to calculate the mass of "di phosphorus pentoxide."
Since you're already given 1.2x10^25 atoms, you start with that. You need to cancel out the atoms, so you need Avogadro's number as shown in the image.
(This has nothing to do with the problem) But in case if you're wondering, the "di" in phosphorus means there's 2 phosphorus and the "pent" means that there are 5 oxygens. So P2O5. Go to your periodic table, multiply their respective atomic masses. You would multiply phosphorus twice and oxygen 5 times. And add them up to get the overall mass.
I hope this helped!