Chapter 13 - The Chemistry of Solutes and Solutions - Questions for Review and Thought - Topical Questions - Page 605b: 25

$-264.3\,kJ/mol$

Recall that $\Delta_{r}H_{solution}=-lattice\, energy+ \Delta_{r}H(cation\, hydration) +\Delta_{r}H(anion\, hydration)$ $\implies -63.3\,kJ/mol=-(-759\,kJ/mol)+(-558\,kJ/mol)+\Delta_{r}H(I^{-}\, hydration)$ $\implies \Delta_{r}H(I^{-}\, hydration)=-63.3\,kJ/mol-759\,kJ/mol+558\,kJ/mol$ $=-264.3\,kJ/mol$