First of you must know the number of moles to know the molecules of any substance. Here moles of (NH4)2SO4 = weight of (NH4)2SO4/molar mass of (NH4)2SO4 = 3/132 = 0.022 moles. Now multiply number of moles with Avogadro's number to get number of molecules or atoms. Therefore, number of molecules in (NH4)2SO4 = 0.022 x 6.02 x 10^23 = 1.3244 x 10^22 molecules. Hope this helps!
