The ionization energy of sodium in $KJmo{l}^{-1}$ if electromagnetic radiation of wavelength 242 nm is just sufficient to ionize sodium atom is  

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

Energy to just remove 1 electron from 1 mol sodium atom = Energy for removal of 1e– from 1 atom
 Energy for removal of 1e– from 1 mol atoms $E=\frac{hc}{\lambda }\times N_A$ 
$E=\frac{6.62\times {10}^{-34}\times 3\times {10}^8\times 6.023\times {10}^{23}}{242\times {10}^{-9}}$ 
$E=494.28kJ/mol=494$