First, we need to figure out the molar mass of P2O5.
P x 2: 30.97g x 2 = 61.94g
O x 5: 16.00g x 5 = 80.00g
+ 141.94g
0.450g x 1 mole/141.94g = 0.00317 moles = 3.17 x 10^3 moles
Now we are going to use Avogadro's number (=6.02 x 10^23)
(3.17 x 10^3 moles) x (6.02 x 10^23 atoms) / (1 mole) = 1.91 x 10^27 atoms
