= Solution
The passive upper limiting <membrane potential> for a purely <sodium>-selective pathway is its <Nernst equilibrium potential>. Equal inside and outside <electrochemical potentials>, using ideal <concentrations>, require
$$
RT\ln\frac{c_{\rm in}}{c_{\rm out}}+FV=0,
\qquad
\boxed{E_{\rm Na}=\frac{RT}{F}\ln\frac{145}{15}
=+60.6\,\mathrm{mV}\quad(T=310.15\,\mathrm K).}
$$
This is the maximum absolute inside-minus-outside voltage under the stated passive assumptions. If the reference resting <membrane potential> is $-60\,\mathrm{mV}$, the corresponding increase is about $121\,\mathrm{mV}$.
Actual <action potentials> usually peak below $E_{\rm Na}$ because <voltage-gated sodium channels> undergo <ion-channel inactivation>, while outward <potassium> and leak <electric currents> oppose inward <sodium> <electric current>. Finite <membrane capacitance> and finite available time also prevent instantaneous equilibration. At $V=E_{\rm Na}$, the <sodium> driving force itself is zero.
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