Na/K pump

A marine protozoan is known to pump sodium ions outward by a simple ATP-driven Na+ pump that operates independently of potassium ions.  The intracellular concentrations of ATP, ADP, and Pi are 20, 2, and 1 mM, respectively, and the membrane potential is -75 mV.
a. assuming that the pump transports three sodium ions outward per molecule of ATP hydrolyzed, what is the lowest internal sodium ion concentration that can be maintained at 25 oC when the external sodium ion concentration is 150 mM?
b. if you were dealing with an uncharged molecule rather than an ion, would your answer for part a be higher or lower, assuming all other conditions remained the same?  Explain.