$$F=k\frac{q_1q_2}{r^2}$$
where \(F\) is the force, \(k\) is Coulomb's constant \((8.98\times10^9\text{ N}\cdot\text{m}^2/\text{C}^2)\), \(q_1\) and \(q_2\) are the magnitudes of the charges, and \(r\) is the distance between the charges.
Sodium and potassium both form +1 ion so \(q_1\) is the same for both. Chloride ion carries -1 charge so \(q_2\) is also same. The difference is only due to the distance \(r\). The ionic radius of \(Na^+\) and \(K^+\) are \(0.97 \AA\) and \(1.33\AA\), respectively. Since the radius of \(K^+\) ion is larger, the distance between \(K^+\) and \(Cl^-\) is longer than the distance between \(Na^+\) and \(Cl^-\).
According to Coulomb's law, the force between two charges is inversely proportional to the square of the distance between them. Therefore, a stronger force of attraction is expected between \(Na^+\) and \(Cl^-\) ions.
Hence, it should be more difficult to pull apart a sodium ion from chloride ion than a potassium ion from chloride ion.
