The electron affinity of $\mathrm{Cl}$ when $1.0\mathrm{g}$ of $\mathrm{Cl}$ atom is converted to ${\mathrm{Cl}}^{-} \left(\mathrm{g}\right)$ liberate $9.83 \mathrm{kJ}$energy is:

$\mathrm{Cl}\left(\mathrm{g}\right)+1{\mathrm{e}}^{-}⟶{\mathrm{Cl}}^{-}\left(\mathrm{g}\right)$

$1\mathrm{g}$ of $\mathrm{Cl}$-atoms liberate energy

                $=9.83 \mathrm{kJ}$

$\therefore 35.5\mathrm{g}$ of $\mathrm{Cl}$-atoms liberate energy

               $=9.83\times 35.5=349\mathrm{kJ}$

$\therefore$ Electron affinity of $\mathrm{Cl}$

                    $=-349 \mathrm{kJ}$