The standard heat of formation, in kcal/mol, of $Ba^{2+}$ is: Given: Standard heat of formation of SO₄²⁻(aq) = -216 kcal/mol, standard heat of crystallization of BaSO₄(s) = -4.5 kcal/mol, standard heat of formation of BaSO₄(s) = -349 kcal/mol.
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Always apply Hess's Law when you have multiple reactions and want to find the heat of formation of an intermediate species.