Given,
The ionisation energy of hydrogen atom = 13.6 eV
We know,
Ionisation energy = 13.6Z2n2 eV
[Since ionisation energy is defined for ground state only, thus n=1]
Thus,
Ionisation enrgy (I.E)∝Z2
∴I.EHI.ELi2+=Z2HZ2Li2+
⇒I.ELi2+=(I.E)HZ2Li2+Z2H
Therefore, the ionisation energy of Li2+ ion
= 13.6×3212 eV=122.4 eV