An electromagnetic radiation of wavelength 242 nm is just sufficient to ionize a sodium atom. Calculate the ionization energy of sodium in kJmol-1

From E= hv or hc/ λ, we have λ = 242nm or 242 x 10-9m

C = 3 x 108 m/s

h = 6.62 x 10-34 Js

Now putting these values in the equation we get

E= (6.62 x 10-34 Js x 3 x 108 m/s) / (242 x 10-9m)

= 0.0821 x 10-17 J/atom

Or

E = (0.0821 x 10-17 ) / (1000 x 6.02 x 1023)

= 494 KJ/mol

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