How to calculate pH The concentration of oxonium ions in solution can be expressed in terms of the pH scale. The pH of a solution is the logarithm to base 10 of the reciprocal of the numerical value of the oxonium ion concentration: pH = lg 1/H30+ = -lg [H30+] = -lg [H+] The pH of a neutral solution can be calculated directly from the ionic product of water. Kw = [H3O+] = [OH-] OR [H+] = [OH-] [H+] = 10-7 pH = 7 for neutral solutions. Acidic solutions have a pH less than 7 at 25oC. pH values of bases The concentration of hydroxide ions in a solution can be expressed in terms of pOH. This is given by: pOH = -lg[OH-] It is possible to write an expression relating pH and pOH as pKw = pH + pOH At 25oC: pH + pOH = 14 Calculation examples Calculate the pH value of 0.001 moldm-3 solution of NaOH at 25oC. [OH-] = 10-3 Therefore: p[OH-] = 3 If pH + 3 = 14, then pH = 11 Calculate the pH of a solution of 0.1M hydrochloric acid at 25oC: [H+] = 10-1 moldm-3 pH = -log10-1 = 1