1.49 x 10^22 atoms
First determine the molar mass of P2O5 Atomic mass of Phosphorus = 30.973762 Atomic mass of Oxygen = 15.999 Molar mass of P2O5 = 2 * 30.973762 + 5 * 15.999 = 141.9425 Now determine how many moles of P2O5 you have 0.500 / 141.9425 = 0.003523 Since there's a total of 7 atoms per molecule of P2O5, multiply the number of moles by 7 to get the number of moles of atoms you have. 7 * 0.003523 = 0.024661 Finally, multiply by avogadro's number (6.0221409 x 10^23) to get the total number of atoms. 0.024661 * 6.0221409 x 10^23 = 1.48512 x 10^22