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. We calculate as follows:
2.0*10^18 molecules of water ( 1 mol / 6.022 x 10^23 molecules ) = 3.32x10^-6 mol water