[mariamendizabal]

At 37 °C the equilibrium concentration of Ca2+ inside a cell is 0.077 M and outside is 0.038 M. The cell membrane is permeable to Ca2+. The only ion present in addition to Ca2+ and Cl– is a protein (Prin+/–) 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. What is the concentration of Prin+/– (in M)?

[Arkcon]

Oh, this one is a fun challenge.  For starters, can you try to diagram for yourself, the concentrations and the charges inside and outside of the cell?

[mariamendizabal]

I know the charge of the protein is negative, but I don't know how to approach this problem, because they are already giving me the concentrations at equilibrium, rather than letting me figure out what changes, do I need to go backwards? I know at some point I should use the Donnan equilibrium formula
[Cl- ]in * [Ca+2 ]in*[Pr] = [Cl- ]out * [Ca+2 ]out, is that correct?

My table looked like this, but I know it's wrong
Inside                  l            Outside
Ca2+: 0.070M+x  l            Ca2+: 0.039 M-x
Pr                        l
Cl-: x                   l             Cl-: b-x