[Rutherford]

If the volume doesn't change then there wouldn't be any movement at all.

[Borek]

If the volume doesn't change then there wouldn't be any movement at all.

Sigh. That was a rhetoric question. Is it possible for the mass to change but for the volume to remain constant?

[Rutherford]

Yes, it has to change as the ions are smaller than atoms, but again, it will change at both ends so it is m₁+m₂ or 2m.
I missed the example, it should be the cathode. So I imagine that there is only a cathode and at the cathode, the following process happens: Hg²⁺+2e^-  Hg as Hg is bigger than Hg²⁺ the volume  increases, because the solution volume decreases, but the solution won't move. 2 processes cause the move.

[Borek]

Yes, it has to change as the ions are smaller than atoms

It is completely unrelated, surface moves because mercury gets oxidized and becomes dissolved, so it leaves metallic phase and enters aqueous phase.

You are wrong on so many levels I give up. It doesn't make you right, but it gives me a free evening 

[Rutherford]

If it only gets oxidized and becomes dissolved then the solution in the middle should just increase in volume.
Okay, I simply can't understand why the move depends on only 1 process, but not of both (oxidation and reduction). If no explanation, then I leave this problem .

[Rutherford]

I think I got it now.
"...surface moves because mercury gets oxidized and becomes dissolved, so it leaves metallic phase and enters aqueous phase. "
This makes the solution in the middle expand by 3.11cm in one direction, so it can actually move because of one process. The other process causes just that the solution contracts in the other direction by 3.11cm so it has the same volume at the end of these processes.
Borek, I am really sorry for the disturbance, I just couldn't figure it out yesterday  . I hope you understand.

[Borek]

No problem, it happens. Leaving the problem for the night usually helps.

[Schrödinger]

My method was the same. Except that I thought the equation would be :
Hg₂²⁺ + 2e -->  2Hg

-.-!