Ncert Exercises & Solutions

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

Energy of sodium (E) = \frac{N_{A} hc}{λ}

= \frac{(6.023 x 10^{23} {mol}^{- 1}) (6.626 x 10^{- 34} Js) (3 x 10^{8} m / s)}{242 x 10^{- 9} m}

= 4.947 \times 105 J {mol}^{- 1}

= 494.7 \times 103 J {mol}^{- 1}

= 494 kJ {mol}^{- 1}

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