The standard heat of formation, in kcal/mol of $Ba^{2+}$ is :$\\$ [Given : standard heat of formation of $\mathrm{SO}_4^{2-}$ ion (aq) = –216 kcal/mol, standard heat of crystallisation of $BaSO_4$(s) = –4.5 kcal/mol, standard heat of formation of $BaSO_4$(s) = – 349 kcal/mol]