Moles of N2O5 = number of molecules/Avogadros number = 4.04 x 1020/6.022 x 1023 = 0.00067087 mol Mass of N2O5 = moles x molar mass of N2O5 = 0.00067087 x 108.01 = 0.0725 g = 7.25 x 10-2 g Solution Moles of N2O5 = number of molecules/Avogadros number = 4.04 x 1020/6.022 x 1023 = 0.00067087 mol Mass of N2O5 = moles x molar mass of N2O5 = 0.00067087 x 108.01 = 0.0725 g = 7.25 x 10-2 g.