we need to know the molar mass of CaSO4 · 2H2O. The mass of CaSO4 is 136 g/mol, and adding to that 2 water molecules (18 g/mol each) gives 172 g/mol. Therefore, we have 7.3x10^-3 / 172 = 4.24x10^-5 mol of CaSO4 · 2H2O.
Now, in one "unit" of CaSO4 · 2H2O, there are 6 O atoms. If we have 4.24x10^-5 mol of the "unit", we have 6 times as many O atoms, or 2.54x10^-4 mol of O atoms.
Finally, we can figure out the actual number of O atoms by multiplying by Avogadro's number: 2.54x10^-4 x 6.02x10^23 = 1.53x10^20 O atom
