From the periodic table:
molecular mass of P = 30.9 grams
molecular mass of O = 16 grams
Therefore:
molar mass of P2O5 = 2(30.9) + 5(16) = 141.8 grams
Number of moles = mass / molar mass = 0.22/141.8 = 1.55 x 10^-3 moles
One mole of any substance contains Avogadro's number of atoms. Therefore, number of atoms in 1.55 x 10^-3 moles is:
1.55 x 10^-3 x 6.022 x 10^-3 = 9.34 x 10^20 atoms
