At the temperature of thirty seven degree Celsius the equilibrium concentration of Ca2+ inside a cell = 0.085 M and outside = 0.042 M. The cell membrane is permeable to Ca2+. The only ion present in addition to Ca2+ and Cl- is a protein (Pr +/-) inside the cell. The protein holds a single positive or negative charge. The Cl- is at equilibrium across the membrane while the protein is inside the cell and cannot permeate the membrane. Determine the concentration of Pr+/- (in M)?