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:
2.30 g NH3 (1 mol / 17.03 g ) (6.022 x 10^23 molecules / 1 mol ) = 8.13x10^22 molecules