Question

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

• A. 498.65 kJ/mol
• B. 398.50 kJ/mol
• C. 494.65 kJ/mol
• D. 200.50 kJ/mol

Answer 1

Answer: (C)

$\lambda =242\mathrm{nm}=242\times 10^{-9}\mathrm{m}$
I.E. $=\frac{hc}{\lambda }=\frac{6.626\times 10^{-34}\times 3\times 10^8}{242\times 10^{-9}}$
$=8.214\times 10^{-19}\mathrm{J}/$ atom
Ionisation energy of sodium in $\mathrm{kJ}{\mathrm{mol}}^{-1}$
$=8.214\times 10^{-19}\times 10^{-3}\times 6.022\times 10^{23}$
$=494.65\mathrm{kJ}/\mathrm{mol}$