364
Acid-Base Equilibria
5. Weak base (see p 353). The equilibrium reaction is
B + H 2 O «* BH
+ + OH[BH
+ ] = [OH-]
[OH-][BH
+
] = [OH-]
2 = [B]K,
[OH-] = V[B]K,
6. Weak acid and its salt, or weak base and its salt (see p 356). Here the
common-ion effect is involved; the solutions are buffers. The equilibria involved are the same as in type 4 or 5 above, except that for the acid solution
[H
+ ] * [A"], and for the basic solution [OH'] *[BH
+ ]. After first calculating the salt concentration and the concentration of the corresponding acid
or base, we find
,
[HA] v
[acid] „ ., ,
. , ...
[H
+ ] = K, = - r — -r-f- K, for the acid solution
[A j
[saltj
[OH"] = j - K, =
Lijrl J
[saltj
for the base
7. Salt of a strong acid and a strong base (see p 358). Salts of this type do
not hydrolyze. Consequently, the pH of such solutions is 7.00, the same as that
of pure water.
8. Salt of a weak acid and a strong base (see p 359). This is a hydrolysis
equilibrium reaction:
A- + H 2 O «s HA +
[OH-] = [HA]
[OH-]' = [A-]*, =
V
r A-iin-14
1
] " —, where K, is for the acid, HA
A,
9. Salt of a weak base and a strong acid (see p 359). This is a hydrolysis
equilibrium reaction:
BH
+ + H 2 O <=s B + H 3 O
+
[H
+ ] = [B]