Consider an electrochemical cell: A(s) | An+ (aq, 2 M) || B2n+ (aq, 1 M) | B(s). The value of H? for the cell reaction is twice that of ?G? at 300 K. If the emf of the cell is zero, the ?S? (in J K?1 mol?1) of the cell reaction per mole of B formed at 300 K is ____. (Given: ln(2) = 0.7, R (universal gas constant) = 8.3 J K?1 mol?1. H, S and G are enthalpies, entropy and Gibbs energy, respectively.)