F or the electrochemical cell, $\mathrm{Mg}(\mathrm{s})\left|\mathrm{Mg}^{2+}(\mathrm{aq}, 1 \mathrm{M}) \| \mathrm{Cu}^{2+}(\mathrm{aq}, 1 \mathrm{M})\right| \mathrm{Cu}(\mathrm{s})$ the standard emf of the cell is 2.70 V at 300 K . When the concentration of $\mathrm{Mg}^{2+}$ is changed to $\boldsymbol{x} \mathrm{M}$, the cell potential changes to 2.67 V at 300 K . The value of $\boldsymbol{x}$ is $\_\_\_\_$ . (given, $\frac{\mathrm{F}}{\mathrm{R}}=11500 \mathrm{~K} \mathrm{~V}^{-1}$, where $F$ is the Faraday constant and $R$ is the gas constant, $\ln (10)=2.30$ )