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Percent titrated

100%

FIGURE 2-18 Comparison of the titration curves of three weak acids.

Shown here are the titration curves for CH3COOH, H2PO/f, and NH4~. The predominant ionic forms at designated points in the titration are given in boxes. The regions of buffering capacity are indicated at the right. Conjugate acid-base pairs are effective buffers between approximately 10% and 90% neutralization of the proton-donor species.

ready half dissociated at pH 4.76. Dihydrogen phosphate loses a proton less readily, being half dissociated at pH 6.86. Ammonium ion is the weakest acid of the three and does not become half dissociated until pH 9.25.

The most important point about the titration curve of a weak acid is that it shows graphically that a weak acid and its anion—a conjugate acid-base pair—can act as a buffer.

SUMMARY 2.2 Ionization of Water, Weak Acids, and Weak Bases

■ Pure water ionizes slightly, forming equal numbers of hydrogen ions (hydronium ions, H3O+) and hydroxide ions. The extent of ionization is described by an equilibrium constant, Keq = [H+][OH~ [H2O]

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