The Foundations of Chemistry

Strong bases are ionic compounds in

the solid state.

The weak bases are molecular

substances that dissolve in water to

give slightly basic solutions; they are

sometimes called molecular bases.

4-2 Aqueous Solutions: An Introduction 133

in Table 4-7. They are the hydroxides of the Group IA metals and the heavier members
of Group IIA. The equation for the dissociation of sodium hydroxide in water is typical.
Similar equations can be written for other strong bases.

H 2 O

NaOH(s)8888nNa
(aq)
OH(aq) (to completion)

Other metals form ionic hydroxides, but these are so sparingly soluble in water that
they cannot produce strongly basic solutions. They are called insoluble basesor some-
times sparingly soluble bases. Typical examples include Cu(OH) 2 , Zn(OH) 2 , Fe(OH) 2 ,
and Fe(OH) 3.
Common weak basesare molecular substances that are soluble in water but form only
low concentrations of ions in solution. The most common weak base is ammonia, NH 3.

NH 3 (aq)
H 2 O(
) 34 NH 4 
(aq)
OH(aq) (reversible)

Closely related N-containing compounds, the amines,such as methylamine, CH 3 NH 2 ,
and aniline, C 6 H 5 NH 2 , are also weak bases. Nicotine (found in tobacco) and caffeine
(found in coffee, tea, and cola drinks) are naturally occurring amines.

EXAMPLE 4-2 Classifying Bases

From the following lists, choose (i) the strong bases, (ii) the insoluble bases, and (iii) the weak
bases. (a) NaOH, Cu(OH) 2 , Pb(OH) 2 , Ba(OH) 2 ; (b) Fe(OH) 3 , KOH, Mg(OH) 2 , Sr(OH) 2 ,
NH 3.

Plan

(i) We recall that Table 4-7 lists the common strong bases.(ii) Other common metal hydroxides
are assumed to be insoluble bases.(iii) Ammonia and closely related nitrogen-containing
compounds, the amines, are the common weak bases.

Solution

(a) (i) The strong bases are NaOH and Ba(OH) 2 , so

(ii) the insoluble bases are Cu(OH) 2 and Pb(OH) 2.

(b) (i) The strong bases are KOH and Sr(OH) 2 , so

(ii) the insoluble bases are Fe(OH) 3 and Mg(OH) 2 , and

(iii) the weak base is NH 3.

You should now work Exercises 20 and 22.

TABLE 4-7 Common Strong Bases

Group IA Group IIA

LiOH lithium hydroxide

NaOH sodium hydroxide

KOH potassium hydroxide Ca(OH) 2 calcium hydroxide

RbOH rubidium hydroxide Sr(OH) 2 strontium hydroxide

CsOH cesium hydroxide Ba(OH) 2 barium hydroxide