Question

Electromagnetic radiation of wavelength 242 nm is just sufficient to ionize the sodium atom. Calculate the ionization energy of sodium in KJ mol^-1.

Answer 1

Wavelength of radiation, λ = 242 nm = 242 x 10^-9 m

Energy (per mole) of the photons = {N_A.hc}/{λ}

= {6.023 x 10²³ mol^-1 x 6.626 x 10^-24 Js x 3 x 10⁸ ms^-1}/{242 x 10^-9 m}

= 4.95 x 10⁵ J mol^-1

= 494 kJ mol^-1

This energy is equal to the ionization energy of sodium. Therefore, Ionisation energy of sodium = 495 kJ mol^-1