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.

$ EMF (millivolts) = \pm 61 * log \frac{C_{1}}{C_{2}}$

Using Nernst equation (reproduced from 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.

  • 3
    $\begingroup$ Can you try to format your question to be more readable? As-is it is a complete mess. Show your thinking step-by-step: which numbers actually matter to the answer, what is the equation you are using, etc. And remove the bits about your professor and your university being shut down - yes, that sort of thing is affecting all of us, but it doesn't change the on/offtopicness of questions here. $\endgroup$
    – Bryan Krause
    Mar 23, 2020 at 16:49
  • 3
    $\begingroup$ Thanks for the edits, this is much better looking! One last thing: can you write out the entire Nernst equation, and show where you have substituted numbers for variables? You're very close. $\endgroup$
    – Bryan Krause
    Mar 23, 2020 at 22:33

1 Answer 1


The Nernst equation should be:

$\frac{RT}{zF}\ln\left(\frac{\mathrm{Out}}{\mathrm{In}}\right) = 2.3 \frac{RT}{zF}\log_{10}\left(\frac{\mathrm{Out}}{\mathrm{In}}\right)$

You substituted for 2.3 RT/F (approximately 60 or 61 mV: use 60 because your instructor asked you to), but forgot about z. Calcium is divalent (and positive) so for calcium z=2. Substitute this and you can see how you get your answer.

(Also note the "out/in" which is how you get negative numbers when z is positive and out < in )


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.