ChemTopper Free Chemistry Help

Here it is
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Na⁺ is more smaller in size than K ⁺ which results in more hydration or  solvation of Na⁺  ions in water
Due to this  overall size of Na⁺  increases in the hydrated form and hence the mobility decreases
Mobility depends on the  size of the ion in solvated form
More bigger size lesser mobility
Smaller size more mobility