Suppose a chloride ion and a sodium ion are separated by a center-center distance of 5Å5Å. Is the interaction energy (the energy required to pull them infinitely far apart) predicted to be larger if the medium between them is water, or if it is n-pentane?

The water is a polar solvent which means it strongly interact with the ionic species dissolved in it. The reason is due to the formation of Hydrogen bonding. Hence, the ionic species are known to be stabilized by solvation.

The H- bonding is the interaction of H-atom of solvent molecules with the electronegative atom of same molecule or, solute species. Ionic solutes dissolve by forming H- bonding.

But in case n- pentane which is a non polar and lack of H-atom attached to electronegative atom. Hence, there is no H- bonding.

Thus, the energy required to separate Na+ and Cl- ion would be huge in water compared to n-pentane.