788 kJ of energy is required to separate one mole of solid NaCl into one mole of Na+(g) and one mole of Cl-(g) to an infinite distance. It means that

From the definition, the lattice enthalpy of an ionic solid is defined as the energy required to completely separate one mole of a solid ionic compound into gaseous constituent ions. Thus, 788 kJmol-1 is the lattice enthalpy of NaCl.