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Electromagnetic radiations of wavelength 242nm is just sufficient to ionize sodium atom.Calculate the ionization energy of sodium in KJ/mol.

$\begin{array}{1 1}(a)\;546.3KJ/mol&(b)\;438.2KJ/mol\\(c)\;342.3KJ/mol&(d)\;494.5KJ/mol\end{array}$

Energy associated with a photon of 242nm=$\large\frac{6.625\times 10^{-34}\times 3\times 10^8}{242\times 10^{-9}}$
$\Rightarrow 8.21\times 10^{-19}J$
$1$ atom of Na for ionization requires =$8.21\times 10^{-19}\times 6.023\times 10^{23}$
$\Rightarrow 49.45\times 10^4J$
$\Rightarrow 494.5$KJ/mol
Hence (d) is the correct answer.