For the electrochemical cell, Mg(s) | Mg²⁺ (aq, 1 M) || Cu²⁺ (aq, 1 M) | Cu(s), the standard emf of the cell is 2.70 V at 300 K. When the concentration of Mg²⁺ is changed to x M, the cell potential changes to 2.67 V at 300 K. The value of x is ____. (given, F/R = 11500 K V⁻¹, where F is the Faraday constant and R is the gas constant, ln(10) = 2.30)