The Standard Enthalpy of Solution, DH°sol, 298, is the heat change which takes place when one mole of a solute is completely dissolved in a solvent to form a solution of concentration 1 mol dm-3, measured under standard conditions.
Enthalpy of Solution can be measured experimentally. It can also be calculated; it is the sum of two imaginary steps: the reverse of the lattice enthalpy plus the sum of the hydration enthalpies of the ions.
DH°sol, 298® -DH°LE, 298 + (DH°hyd, 298 (anion) + DH°hyd, 298 (cation))
This relationship is illustrated in the enthalpy diagram below for sodium chloride:
DH°sol, 298 = --776 + -771 = +5 kJ mol-1
See also Hess's Law | Hydration Enthalpy | Lattice Enthalpy.