3: H2O dissociation, pH, Weak Acids and Bases

I. Dissociation of water

Water dissociates to a very small extent. The concentrations of H⁺ and OH^- in pure water are 10^-7 M (this is one molecule dissociating among 5 X 10⁸).

II. pH

A. Brönsted acids (proton donor or OH^- acceptor) and bases (proton acceptor or OH^-donor)

B. Strong acids and bases completely dissociate in water (HA –> A^- + H⁺ , BOH –> B⁺ OH^- completely)

C. Weak acids and bases - dissociation incomplete, [H⁺] determined by an equilibrium equation.

D. Buffers - mixtures of conjugate acid and conjugate base at ±1 pH unit from pH = pK_a. Resistant to changes in pH in response to small additions of H⁺ or OH^-.

Some take home information

Contributors

• Charles S. Gasser (Department of Molecular & Cellular Biology; UC Davis)