The ionization enthalpy is the amount of energy required to remove an electron from ground state to infinity. The energy of electron in the ground state is −2.18×10−18J
The energy of electron at infinity is zero.
The energy required to remove electron is 0−(−21.8×10−18J)=21.8×10−18J
To remove 1 mole of electrons the amount of energy required is 2.18×10−18×6×1023=13.08×105J. This is the ionization enthalpy of hydrogen.