http://www.ck12.org Chapter 21. Acids and Bases
Salts Forming Basic Solutions
A salt formed from a weak acid and a strong base will form a solution that has a pH greater than than 7. For example,
sodium acetate could be formed by neutralizing acetic acid with sodium hydroxide. Dissolving sodium acetate in
water will produce sodium ions and acetate ions. The Na+ion is not a strong enough acid or base to donate or accept
any protons, but the acetate ion is a weak base:
CH 3 COO−+ H 2 O CH 3 COOH + OH−
Although this equilibrium favors the reactants (acetate is still a weak base), there will still be some additional
hydroxide produced, skewing the H+/ OH−balance and increasing the pH of the solution. The equilibrium constant
for this reaction can be written as follows:
Kb=[CH^3 COOH][OH
−]
[CH 3 COO−] =^5.^6 ×^10− 10Salts Forming Acidic Solutions
A salt formed from a strong acid and a weak base will give an acidic solution when dissolved in water. For example,
NH 4 Cl, which is formed by neutralizing the weak base ammonia with HCl, would dissociate into the following ions:
NH 4 Cl
H 2 O
→NH 4 ++ Cl−
The ammonium ion then acts as a weak acid, while the chloride ion is not a strong enough base to affect the pH of
the solution.
NH 4 + NH 3 + H+
Calculations involving acidic and basic salts are carried out in the same way as other acid-base equilibrium calcu-
lations. Depending upon the data available, we can calculate equilibrium constants or the pH of a solution with a
known concentration.
Buffer Solutions
Consider the generic equilibrium for the dissociation of an acid:
HA H++ A−
In the reverse direction, the anion A−is acting as a base by accepting a proton. For a given acid (HA), its
deprotonated form (A−) is referred to as itsconjugate base. The conjugate base of a strong acid is an extremely
poor base, which is why the reverse reaction essentially does not proceed. For example, HCl dissociates completely
in water, and the chloride ion (its conjugate base) is not really thought of as a base. In contrast, the conjugate base
of a weak acid is a weak base. For example, the acetate ion, which is the conjugate base of acetic acid, is slightly
basic.
A similar terminology is used when dissolving bases in water. After the base accepts a proton, the resulting molecule
or ion is referred to as itsconjugate acid. For example, the ammonium ion (NH 4 +) is the conjugate acid of ammonia
(NH 3 ).
We can take advantage of the properties of conjugate acids and bases to makebuffersolutions, which resist small
changes in pH when an acid or a base is added to the system. Biological buffers play important roles in maintaining
a constant pH in cells and tissues, which is required in order for many biochemical reactions to proceed efficiently.
Buffers can be generated in the laboratory by making a mixture of both a weak acid and its conjugate base. Consider
the following equilibrium between acetic acid and the acetate ion:
CH 3 COOH H++ CH 3 COO−