The pH scale was designed to make it convenient to express hydrogen ion concentrations that are small in aqueous solutions. The definiton of pH is in terms of base 10 logarithms. pH = -log[H] where (H+) is the hydrogen ion concentration a. If the hydrogen lon concentration in a solution is 9.86 x 10-3 mol/L, the pH is b. If the pH of a solution is 3.688, the hydrogen ion concentration is mol/L