The concentration of potassium ions is 5 mM outside and 100 mM inside, but the membrane is impermeable to these ions. We will keep the intracellular and extracellular fluids electroneutral by making sure that there's an appropriate number of matching negative ions (depicted as ‘A-’ above) on both sides of the membrane. If we measure Em in this new situation, it will be: 0 mV. positive. or negative?