for the cell reaction, ${\mathrm{Cu}}^{2+}\left({\mathrm{C}}_1,\mathrm{aq}\right)+\mathrm{Zn}\left(\mathrm{s}\right)={\mathrm{Zn}}^{2+}\left({\mathrm{C}}_2,\mathrm{aq}\right)+\mathrm{Cu}\left(\mathrm{s}\right)$ of an electrochemical cell, the change in free energy, $\mathrm{\Delta G},$at a given temperature is a function of 

$\mathrm{\Delta G}=-{\mathrm{nE}}^{\circ }\mathrm{F}$
For concentration cell, $\mathrm{E}=\frac{\mathrm{RT}}{\mathrm{nF}}\ln \frac{{\mathrm{C}}_2}{{\mathrm{C}}_1}$

In it R, T, n and F are constant 
$\text{ So }\mathrm{E}\text{ is based upon }\ln {\mathrm{C}}_2/{\mathrm{C}}_1$
Now $\mathrm{\Delta G}=-\mathrm{nEF}=-\mathrm{nF}\times \frac{\mathrm{RT}}{\mathrm{nF}}\ln {\mathrm{C}}_2/{\mathrm{C}}_1$
$=-{\mathrm{RTlnC}}_2/{\mathrm{C}}_1$
At constant temperature  $∆$G is based upon  $\ln \left({\mathrm{C}}_2/{\mathrm{C}}_1\right)$