Calculate Hf  (heat of formation) of AlCl3 (s) knowing that: Al2O3 (s) 3C(graphite) 3Cl2 (g) → 2AlCl3 (s) 3CO(g) H rxn = 53.55 kJ Hf  CO(g) = -110.5 kJ/mol; Hf  Al2O3 (s) = -1673.6 kJ/mol