# Nernst Equation

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

#### masala

##### Full Member
I have been trying to figure out this question for a while and have not been able to do so, can anyone help?

I’m wrong and I think I figured it out. Since the reaction is at equilibrium you sub keq in for log Q. Voltage is zero at equilibrium so E is zero.

The standard calculation method from general chemistry is cumbersome and will take several minutes (I will represent it second). But this question can be determined quickly and easily using PoE and intuition.

PoE and Intuition Method:
First, look at the half reaction table. NADH must be reversed, resulting +0.32 V. Pyruvate remains as is, making it -0.19. The overall redox reaction has E = +0.13. A positive E means that the reaction is favorable as written, so Keq is greater than 1. This eliminates choices A and B.

The easiest way to decide between C and D involves looking at the 0.06/n and 0.13 ratio. The value of n is 2 in this case (the number of electrons exchanged in the reaction), so we have 0.03 and 0.13 in the calculation. 0.03 does not divide into 0.13 evenly, so there is a remainder, which means that the first part of the answer cannot be 1.00. This eliminates choice D and leaves choice C.

If you want a more precise answer, then we can use the Nernst Equation.
Consider that at equilibrium, deltaG = 0, which means that E is also 0. This is also where K = Q, so we can substitute K into the equation for Q and 0 for E.

0 = 0.13 - (0.06/2)logK = 0.13 - 0.03logK

0.03logK = 0.13

so logK = 0.13/0.03 = 4.33​

This means that K = 10^4.33 = 10^0.33 x 10^4.
The log of 2 = 0.30, so 10^0.33 = 2.
This gives us 2 x 10^4.

In all honesty, I think the Math-based solution is far above and beyond the MCAT and would highly recommend the PoE method presented first. I'm not sure why AAMC would put this question on their practice exam, as this calculation seems beyond what they typically do.

Members don't see this ad :)
I'm not sure why AAMC would put this question on their practice exam, as this calculation seems beyond what they typically do.
This is a next step practice test; not AAMC.

It does not appear in AAMC full length practice exams like this. There is a Nernst question but more about the relevance to neurons

1 user
Like Ad2b said, this is definitely not AAMC style. They ask either about the impact of a change in some experiment or a biological application. You should think about things like what would happen to the voltage reading if there were lactate build up or why is the voltage only changing on the millivolt scale and not the volt scale.