Molecular mass of Al2O3 = 27*2 + 16*3 = 102g/mol
so moles in 0.051 = mass/Mm = 0.051/102 = 0.0005 moles
2 Al ions per molecule, so 0.001 moles of Al ions
number of ions = moles x NA where NA is avogadros number = 6.022x10^23
so number of ions = 6.022x10^23 x 0.001 = 6.022x10^20 ions