Question

Electromagnetic radiation of wavelength $242nm$ is just sufficient to ionise the sodium atom. Calculate the ionisation energy in $kJ$ ${mol}^{-1}$

[$h=6.6256\times {10}^{-34}Js$]

Answer 1

$\lambda =242nm=242\times {10}^{-9}m$

$c=3\times {10}^{8}m{s}^{-1}$

$E=hv=h\cdot \frac{c}{\lambda }=6.6256\times {10}^{-34}\times \frac{3\times {10}^8}{242\times {10}^{-9}}$

$=0.082\times {10}^{-17}J=0.082\times {10}^{-20}kJ$

Energy per mole of ionisation $=0.082\times {10}^{-20}\times 6.02\times {10}^{23}$

$=493.6$ $kJ$ ${mol}^{-1}$.

Hence, energy per mole of ionisation $=493.6$ kJ mol ${}^{-1}$.