First you must determine how many moles of P3O5 you have. This is done by using the formula Number of moles (n) = mass in grams of substance (m) /divided by/ Molar mass (M) [this is the sum of the atomic mass of all atoms in the compound]
n = 0.170 / P (31 x 2) + O (16 x 5) n = 0.170 / 142 n = 0.001197 moles
Then you use avagadros number 6.02 x10^23 this is the number of atoms in one mole of any substance. Since you have 0.001197 moles you multiply the number of moles by avagadros number
