(a) Oxycodone hydrochloride is the salt of a weak base and a strong acid, hence ph = 2pKa- 2log c where c = 50 g dm 3 = 0.1232 mol dm pH = 4.45 + 0.45 = 4.90

(b) Benzylpenicillin sodium is the salt of a weak acid and a strong base, hence ph = 2pKw + 2pKa + 2log c where c = 600 mg in 2 cm3 = 0.842 mol dm 3.

(c) Chlorphenamine maleate is the salt of a weak acid and a weak base, hence pH = 1pKw + 1pKa - 2pKb

3.5.8 Preparation of buffer solutions buffering action of a weak acid HA and its ionised salt (for example, NaA) is that the A0 ions from the salt combine with the added H+ ions, removing them from solution as undissociated weak acid:

Added OH0 ions are removed by combination with the weak acid to form undissociated water molecules:

The buffering action of a mixture of a weak base and its salt arises from the removal of H+ ions by the base B to form the salt and removal of OH0 ions by the salt to form undissociated water:

The concentration of buffer components required to maintain a solution at the required pH may be calculated using equation (3.70). Since the acid is weak and therefore only very slightly ionised, the term [HA] in this equation may be equated with the total acid concentration. Similarly, the free A0 ions in solution may be considered to originate entirely from the salt and the term [A0] may be replaced by the salt concentration.

Equation (3.70) now becomes

By similar reasoning, equation (3.72) may be modified to facilitate the calculation of the pH of a solution of a weak base and its salt, giving

Equations (3.83) and (3.84) are often referred to as the Henderson-Hasselbalch equations.

A mixture of a weak acid and its salt (that is, a conjugate base), or a weak base and its conjugate acid, has the ability to reduce the large changes in pH which would otherwise result from the addition of small amounts of acid or alkali to the solution. The reason for the

EXAMPLE 3.9 Buffer solutions

Calculate the amount of sodium acetate to be added to 100 cm3 of a 0.1 mol dm 3 acetic acid solution to prepare a buffer of pH 5.20.

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