 Solution

At a common membrane voltage, an ion is in electrochemical equilibrium only if its own Nernst potential equals that voltage. The four independently calculated equilibrium voltages areThus at $V_m=-90$ mV only chloride is at its listed equilibrium voltage. Potassium has an outward positive-current driving voltage $V_m-E_K=+7.74$ mV. Sodium and calcium have negative driving voltages, $-156.60$ and $-220$ mV respectively, favoring inward positive-ion movement when their channels are open. Chloride has zero net electrochemical drive under these ideal conditions.

A resting potential is not a claim that every permeant chemical species is in thermodynamic equilibrium. A membrane with several electrical conductances can have zero total electric current even though individual ionic electric currents flow and cancel. Pumps maintain the concentration differences by consuming Gibbs free energy. Conversely, a closed channel can prevent substantial ion flux even when the ion's electrochemical potential difference is nonzero: absence of flux due to impermeability is not the same as electrochemical equilibrium. The listed concentrations could not all be passive equilibrium distributions at one voltage because their Nernst potentials differ.