How many total atoms are in 0.510 g of p2o5?

First, we need to calculate the moles of P2O5:

moles of P2O5 = mass of P2O5 / Molar mass of P2O5

when the molar mass = 2*31 + 5*16 = 142 g/mol

by substitution:

moles of P2O5 = 0.51 g / 142 g/mol 

                          = 0.0036 moles 

to know how many molecules =  moles P2O5 * Avogadro's number

                                                   =  0.0036 * 6.02 x 10^23

                                                   = 2.17 x 10^21 molecules P2O5

when each molecule of P2O5 has 2 P atoms and 5 O atoms = 7 atoms

∴ the total atoms = 2.17 x 10^21 * 7atoms = 1.5 x 10^22 atoms