8.1 Introduction to Acids and Bases

Calculate the values of pH and pOH, based on Kw and the concentration of all species present in a neutral solution of water.

• The concentrations of hydronium ion and hydroxide ion are often reported as pH and pOH, respectively. EQN: pH = −log[H3O+] EQN: pOH = −log[OH−] The terms “hydrogen ion” and “hydronium ion” and the symbols H+(aq) and H3O+(aq) are often used interchangeably for the aqueous ion of hydrogen. Hydronium ion and H3O+(aq) are preferred, but H+(aq) is also accepted on the AP Exam.
• Water autoionizes with an equilibrium constant Kw. EQN: Kw = [H3O+][OH−] = 1.0 × 10−14 at 25°C
• In pure water, pH=pOH is called a neutral solution. At 25°C, pKw = 14.0 and thus pH = pOH = 7.0. EQN: pKw = 14 = pH + pOH at 25°C
• The value of Kw is temperature dependent, so the pH of pure, neutral water will deviate from 7.0 at temperatures other than 25°C.