Barrons SAT Subject Test Chemistry, 13th Edition

TIP

Salt interacts with water.

In this hydrolysis reaction the same number of hydrogen ions and of hydroxide
ions is released so that the solution is neutral. This occurs because sodium
hydroxide is a strong base and hydrochloric acid is a strong acid. (There are
charts of relative acid and base strengths to use as references in Chapter 11.)
Because they are both classified as strong, sodium hydroxide and hydrochloric
acid essentially exist as ions in solutions. Therefore, the NaCl solution has an
excess of neither hydrogen nor hydroxide ions, and it will test neutral. Thus, the
salt of a strong acid and a strong base forms a neutral solution when dissolved in
water. However, if Na 2 CO 3 is dissolved, we have:

(2Na+ + CO 3 2−) + 2H 2 O(l) → (2Na+ + 2OH−) + H 2 CO 3

The H 2 CO 3 is written as a single entity because it is a slightly ionized acid, or, in

other words, a weak acid. Since the hydroxide ions are free in the solution, the
solution is basic. Notice that here it was the salt of a strong base and a weak acid
that formed a basic solution. This generalization is true for this type of salt.
If we use ZnCl 2 , which is the salt of a strong acid and a weak base, the

reaction will be:

(Zn2+ + 2Cl−) + H 2 O(l) → (2H+ + 2Cl−) + Zn(OH) 2

In this case the hydroxide ions are held in the weakly ionizing compound while
the hydrogen ions are free to make the solution acidic. In general, then, the salt of
a strong acid and a weak base forms an acid solution by hydrolysis.
The fourth possibility is that of a salt of a weak acid and weak base
dissolving in water. An example would be ammonium carbonate, (NH 4 ) 2 CO 3 ,

which is the salt of a weak base and a weak acid. The hydrolysis reaction would
be:

(NH 4 ) 2 CO 3 + 2H 2 O(l) → 2NH 4 OH + H 2 CO 3

Both the ammonium hydroxide, NH 4 OH, and the carbonic acid, H 2 CO 3 , are

written as nonionized compounds because they are classified as a weak base and
a weak acid, respectively. Therefore, a salt of a weak acid and a weak base forms
a neutral solution since neither hydrogen ion nor hydroxide ion will be present in
excess.