The standard heat of formation, in kcal/mol of $\text{Ba}^{2+}$ is: [Given: standard heat of formation of $\text{SO}_4^{2-}$ ion (aq) $= -216$ kcal/mol, standard heat of crystallisation of $\text{BaSO}_4(s) = -4.5$ kcal/mol, standard heat of formation of $\text{BaSO}_4(s) = -349$ kcal/mol]
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Explanation
$\Delta H_f(\text{BaSO}_4) = \Delta H_f(\text{Ba}^{2+}) + \Delta H_f(\text{SO}_4^{2-}) + \Delta H_{cryst}$. $-349 = x + (-216) + (-4.5)\Rightarrow x = -128.5$ kcal/mol.