The ionization energy of hydrogen atom is 1314 kJ mol−1.The ionization energy of He+ ion is expected to be

The ionization energy of hydrogen atom is $1314 kJ {mol}^{- 1}$ .The ionization energy of ${He}^{+}$ ion is expected to be

Since $E \propto - Z^{2} / n^{2}$ ,we have

$\frac{E_{1} ({He}^{+})}{E_{1} (H)} = \frac{- 4 / 1}{- 1 / 1} = 4 \Rightarrow E_{1} ({He}^{+}) = 4 E_{1} (H)$

Hence,

$IE ({He}^{+}) = 4 IE (H) = 4 \times 1314 kJ {mol}^{- 1} = 5256 kJ {mol}^{- 1}$

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