Entropies of Solvation for Chlorate and Nitrate Anions
Look at the graph on p. 65, it puts the entropy of solvation (298 K) for chlorate and nitrate at about 25 and 21 kJ/mol respectively.  This reference suggests that the entropy term for nitrate is 116 J/(K-mol), multiplying that by 298 K yields 34.6 kJ/mol.