why Na+ ion is expected to have the lowest value of molar conductance on infinite dilution in an aqueous solution?

Na⁺ does not have the lowest molar conductance of all ions. It has lower molar conductance than the ions that are greater in size than Na such as K⁺, Cs⁺ as the molar conductance is inversely proportional to solvation of ions. Smaller the ion, greater will be the solvation and hence lower will be the molar conductance.
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