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Question

An electromagnetic radiation of wavelength 198 nm is sufficient to ionise a metal atom. Calculate the ionisation energy in kJmol1,(Planck constant h=6.6×1034 Js, Avogadro number=6.02×1023

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Solution

According to Planck's quantum theory,
The energy of radiation is inversely proportional to its wavelength which is given by, E=hcλ
Given,
λ=198×109 m
h=6.6×1034 Js
As the electromagnetic radiation is just sufficient to ionize the sodium atom,
Ionization energy = Energy of the photon
E=hcλ
=6.6×1034×3×108198×109
=1018 J
=10×1022 kJ/atom
Thus, for the ionisation of 1 atom, energy = 10×1022 kJ is required.
Energy required per mole = 10×1022 kJ×6.02×1023
=602 kJmol1

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