The electrostatic force between the metal plates of an isolated...

For isolated capacitor $Q =$constant.

Now electric field because of one plate of the capacitor is $E = \dfrac{\sigma}{2{\in}_{0}} = \dfrac{Q}{2A{\in}_{0}}$.
$∴$ Force on 1 metal plate is
$F = QE$
$F = Q\dfrac{\sigma}{2{\in}_{0}} = \dfrac{{Q}^{2}}{2A{\in}_{0}}$
$∴$The electrostatic force is independent of the distance between plates.