The energy required to break one mole $Cl-Cl$  bond in $C{l}_2$  is 242KJ/mol. The longest wavelength of light capable of breaking a single $Cl-Cl$   bond is x nm 
 ($C=3\times {10}^{8}m/s,N_A=6.023\times {10}^{23}mo{l}^{-1},h=6.6\times {10}^{-34}J$ ) The value of x is (nearest integer)