The difference in pH between the internal and external surfaces of the inner mitochondrial membrane is 1.4 pH units (external side acidic). If the membrane potential is 0.06 V (inside negative), the free energy change on transporting 1 mol of protons across the membrane from outside to inside at 298K is -13.8 kJ/mol. How many protons must be transported to provide enough free energy for the synthesis of 1 mol of ATP (assuming standard biochemical conditions)?