Avogadro's number represents the number of units in one mole of any substance. This has the value of 6.022 x 10^23 units / mole. This number can be used to convert the number of atoms or molecules into number of moles.
7.3x10^-3 g of CaSO4 ( 1 mol / 136.14 g ) = 5.36x10^6-5 moles
5.36x10^6-5 moles ( 6.022 x 10^23 formula units / mole ) = 3.23x10^19 formula units