Solutions A and B are separated by a membrane that is permeable to Ca2+ and impermeable to Cl−.
Solution A contains 10 mM CaCl2 , and solution B
contains 1 mM CaCl2.
Assuming that 2.3 RT/F = 60 mV, Ca2+ will be at electrochemical equilibrium when
(A) solution A is +60 mV
(B) solution A is +30 mV
(C) solution A is −60 mV
(D) solution A is −30 mV
(E) solution A is +120 mV
(F) solution A is −120 mV
(G) the Ca2+ concentrations of the two solutions are equal
I eliminated A,B and E because of the +ve sign immediately, and G because it doesn't take into account the electrical potential factor. I also understand the general idea (that Ca2+ ions will cross the membrane but Cl- won't so it will stop when solution A is negative enough for the electrical gradient to be > concentration gradient) but I don't know what to do next. Using Nernst equation (here's the version in my book) I got that the equilibrium potential is about -60 mV (I did -61log(10/1)) But the book's answer is D which makes me think I might have a huge misunderstanding in the principle itself.