The enthalpy change when solid sodium hydroxide NaOH is dissolved into water can be calculated using the following equation:H_solution = H_desorption + H_hydrationWhere H_solution is the molar heat of solution, H_desorption is the desorption enthalpy of NaOH, and H_hydration is the enthalpy change associated with the hydration of the ions.Given that the molar heat of solution H_solution is -44.51 kJ/mol and the desorption enthalpy H_desorption is 44.51 kJ/mol, we can rearrange the equation to solve for H_hydration:H_hydration = H_solution - H_desorptionH_hydration = -44.51 kJ/mol - 44.51 kJ/molH_hydration = -89.02 kJ/molTherefore, the enthalpy change when solid sodium hydroxide is dissolved into water is -89.02 kJ/mol.