Page 93 - Instant notes

Further acids and bases 79

which is equivalent to

−
−
as the conjugate base of an acid, A , is a base, B, and so B≡A . This means that since the
+
conjugate acid of a base, B, is an acid and BH ≡HA these symbols can be used
interchangeably. The base dissociation constant is given by the equation (see Topic C1):

By analogy with K a, the greater the magnitude of K b (and the smaller the size of pK b), the
greater is the base strength, or the ability of the base to accept a proton. The base
−
dissociation constant of any base (B≡A ) can simply be calculated from K a for its
+
conjugate acid (HA≡BH ) using the relationship K aK b=K w or pK a+pK b=pK w=14.00.
This means that tabulation of both pK a values for acids and pK b values for their
conjugate bases is unnecessary and often only pK a values for a range of acids are given.
Also from this relationship, it is clear that as the strength of an acid is increased, the base
strength of its conjugate base is decreased, and as the strength of a base is increased, the
acid strength of its conjugate acid is decreased. This means that as HCl is a strong acid,
−
Cl is a very weak base.
By analogy, the relationship can be derived from
the above equation for the base dissociation equilibrium, which means that pOH=pK b
when the activities of the base and its conjugate acid are equal.

Salt solutions

Acidity constants provide an easy means of predicting whether a solution formed from
dissolving a salt will be acidic or basic. These solutions generally consist of an acid (the
positive ion or cation, see Topic E1) and a base (the negative ion or anion). The pH of the
resulting solution will be determined by the relative strengths of the acid and base, with
the strongest dominating. For example, ammonium hydroxide consists of a relatively
−
weak acid and a strong base (OH ) and on dissociation:

the solution will be basic, as the proton accepting ability of the hydroxide ion (as
measured by its very small pK b value) will dominate the proton donating ability of the
ammonium ion (as measured by its relatively large pK a value). This means that overall,
more hydroxide ions will be present than hydronium ions in solution. In contrast, a
solution of ammonium chloride will be acidic, because on dissociation: