Particles? I assume you mean molecules/atoms per mole, or atoms per species. Anyway..., Ba3(PO4)2
15.5 g Ba3(PO4)2 (1 mol Ba3(PO4)2/601.8 g)(6.022 X 10^23/1 mol Ba3(PO4)2)
= 1.6 X 10^22 "particles" of barium phosphate in 15.5 g Ba3(PO4)2
======Now, to find, say, atoms of barium in Ba3(PO4)2
15.5 g Ba3(PO4)2 (1 mole Ba3(PO4)2/601.8 g)(3 mole Ba/1 mol Ba3(PO4)2)(6.022 X 10^23/1 mol Ba)
= 4.7 X 10^22 atoms of barium in 15.5 g Ba3(PO4)2
======And so on, just don't forget the proper molar ratio.
