Here, EX is the equilibrium potential for ion X (in volts), 
R is the gas constant (1.987 cal/mol-degree), T is the
absolute temperature in kelvins (recall that 0?C 273K), z
is the valence of the ion, F is the Faraday constant (23,062
cal/V-mol), [X]outside is the concentration of X outside 
the cell (in M), and [X]inside is the concentration of X
inside the cell (in M).