The enthalpy change for the ionization of one mole of magnesium in the gas phase can be calculated using the ionization energy and electron affinity values given.Ionization energy is the energy required to remove one mole of electrons from one mole of gaseous atoms, while electron affinity is the energy change when one mole of electrons is added to one mole of gaseous atoms.In this case, we are given the ionization energy of magnesium 738 kJ/mol and the electron affinity of magnesium -50.1 kJ/mol . Since the electron affinity is negative, it means that energy is released when an electron is added to magnesium.The enthalpy change for the ionization of magnesium can be calculated as follows:Enthalpy change = Ionization energy + Electron affinityEnthalpy change = 738 kJ/mol + -50.1 kJ/mol Enthalpy change = 738 kJ/mol - 50.1 kJ/molEnthalpy change = 687.9 kJ/molTherefore, the enthalpy change for the ionization of one mole of magnesium in the gas phase is 687.9 kJ/mol.