If the Ionization potential (I.P.) of Na is 5.48 eV. The I.P. of K will be     

 in a group, from top to bottom ionization potential, I.P values decreased.

   Both sodium, Na and potassium, K elements belong to same group(IA).

    For Sodium (Na), $\mathrm{I}.\mathrm{P}=5.48\mathrm{eV}$; For Potassium (K),$\mathrm{I}.\mathrm{P}=4.34\mathrm{eV}$