The energy required to break one mole of $$Cl - Cl$$  bonds in $$C{l_2}$$  is $$242\,kJ\,mo{l^{ - 1}}.$$   The longest wavelength of light capable of breaking a single $$Cl - Cl$$  bond is
( $${c = 3 \times {{10}^8}m{s^{ - 1}}}$$     and $${{N_A} = 6.02 \times {{10}^{23}}mo{l^{ - 1}}}$$     )

Solution :
Energy required to break one mole of $$Cl - Cl$$  bonds in $$C{l_2}$$
$$\eqalign{ & = \frac{{242 \times {{10}^3}}}{{6.023 \times {{10}^{23}}}} = \frac{{hc}}{\lambda } = \frac{{6.626 \times {{10}^{ - 34}} \times 3 \times {{10}^8}}}{\lambda } \cr & \therefore \,\,\,\lambda = \frac{{6.626 \times {{10}^{ - 34}} \times 3 \times {{10}^8} \times 6.023 \times {{10}^{23}}}}{{242 \times {{10}^8}}} \cr & = 0.4947 \times {10^{ - 6}}m \cr & = 494.7\,nm \cr} $$