Final answer:
Given the question's scenario where the membrane is impermeable to potassium ions, the membrane potential (Em) cannot be directly determined. However, under normal physiological conditions where the membrane is permeable to ions, the resting membrane potential is typically around -70 mV, due to the differential ionic distribution and ion channel permeability.
Step-by-step explanation:
If the concentration of potassium ions is 5 mM outside and 100 mM inside, and the membrane is impermeable to these ions, the membrane potential will not be immediately affected by this concentration difference since the membrane is not allowing the movement of potassium ions.
However, typically, when a membrane is permeable to potassium ions, the resting membrane potential is close to the equilibrium potential of potassium due to its high permeability through its channels in the neuronal cell membrane. This potential is called the resting membrane potential, which is most commonly around -70 mV. The resting membrane potential results from the differential distribution of ions (especially Na+ and K+) across the membrane and the selective permeability for these ions, with more leakage channels for K+ than Na+ and the active transport via the Na+/K+ pump.
Because of these factors, the inside of the cell tends to be negatively charged relative to the outside. Given the provided ion concentrations and the properties of the cell membrane described, if the membrane were permeable to potassium, the resting membrane potential would likely be around -70 mV.