Page 24 - Pharmaceutical analytical chemistry |

A- + H2O ↔ HA + OH-

Multiplication of one equilibrium constant expression by the other gives

This relationship indicates that if an acid is weak, its conjugate base must
be strong and vice versa, and

 pH calculations for different solutions

1) Solutions of strong acids and strong bases
Since strong acids and strong bases are completely ionized in solvents,
the calculation of pH or pOH of such reagents is simple, so the [H+] or
[OH-] is directly related to the concentration of the substance.

2) Solutions of weak acids and weak bases
Since weak acids and weak bases are only partially ionized in solution,
the [H+] and [OH¯] in such solutions are less than the molar
concentration of the reagent. The calculation of pH or pOH requires
knowledge of the concentration of the substance as well as the magnitude
of its ionization constant.

a. Calculation of the pH of a solution of a weak monoprotic acid

HA ↔ H+ + A-

If the acid is pure, the ionization gives equal concentrations of H+ and A-,
i.e. [H+] = [A-].

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