The bond dissociation energy is the energy required to break a mole of bonds in a substance. In this case, we are given the energy required to break a single Cl-Cl bond in chlorine gas Cl2 , which is 243 kJ.To find the bond dissociation energy for a mole of Cl-Cl bonds, we need to multiply the energy required to break a single bond by Avogadro's number 6.022 x 10^23 , which represents the number of particles in a mole.Bond dissociation energy kJ/mol = Energy to break a single bond kJ Avogadro's number mol^-1 However, the given energy 243 kJ is already for one mole of Cl-Cl bonds, so there is no need to multiply by Avogadro's number.Bond dissociation energy kJ/mol = 243 kJTherefore, the bond dissociation energy of chlorine gas Cl2 is 243 kJ/mol.