Chapter 16 Text

16.6 Weak Acids 627
Solve: (a) NaOH dissociates in water to give one OH ion per formula unit. Therefore,
the OH~ concentration for the solution in (a) equals the stated concentration of NaOH,
namely 0.028 M.
Method 1:
_ 1-0 X IP"14
[H ] ~ 0.028
Method 2:
= 3.57 x 10~53 M pH - -log{3.57 x lO"13) = 12.45
pOH = -log(0.028) = 1.55 pH - 14.00 - pOH = 12.45
(b) Ca(OH)2 is a strong base that dissociates in water to give two OH~ ions per
formula unit. Thus, the concentration of OH~(aq) for the solution in part (b) is
2 X (0.0011 M) - 0.0022 M.
Method 1:
1 n v
[H+]-
0.0022
= 4.55 X ID"12 M
Method 2:
pOH = -log(0.0022) - 2.66
pH = -log(4.55 x 10~12) = 11.34
pH = 14.00 - pOH = 11.34
PRACTICE EXERCISE
What is the concentration of a solution of (a) KOH for which the pH is 11 .f
(b) Ca(OH)2 for which the pH is 11.68?
Answers: (a) 7.8 X 10~3 M; (b) 2.4 X 10"3 M
Although all the hydroxides of the alkali metals (group 1A) are strong electrolytes,
LiOH, RbOH, and CsOH are not commonly encountered in the laboratory.
The hydroxides of the heavier alkaline earth metals, Ca(OH)2, Sr(OH)2,
and Ba(OH)2 , are also strong electrolytes. They have limited solubilities, however,
so they are used only when high solubility is not critical.
Strongly basic solutions are also created by certain substances that react with
water to form OH~(aq). The most common of these contain the oxide ion. Ionic
metal oxides, especially Na2O and CaO, are often used in industry when a strong
base is needed. Each mole of O2~ reacts with water to form 2 mol of OH~, leaving
virtually no O2~ remaining in the solution:
O2-(aq) + H2O(0 - > 2OH~(aq) J [16.22]
Thus, a solution formed by dissolving 0.010 rnol of Na2O(s) inienough water to
form 1.0 L of solution will have [OH~] = 0.020 M and a pH of 12.30.
Ionic hydrides and nitrides also react with H2O to form OH~:
H2O(0 - > H2(g) + OrT^) [16.23]
3H2O(/) - > NH3(oj) + 3OH~(aq) [16.24]
Because the anions O2~, H~, and N3~ are stronger bases than OH~ (the conjugate
base of H2O), they are able to remove a proton from H2O.
16.6 Weak Acids _
Most acidic substances are weak acids and are therefore only partially ionized in
aqueous solution. We can use the equilibrium constant for the ionization reaction
to express the extent to which a weak acid ionizes. If we represent a general weak
acid as HA, we can write the equation for its ionization reaction in either of the
following ways, depending on whether the hydrated proton is represented as
or
or
H2O(/)
HA(aiy)
[16.25]
[16.26]
Students often confuse a weak add
(small Ka) with a dilute add (low
concentration).
|ohn J. Fortman, "Pictorial Analogies
XI: Concentrations and Acidity of
Solutions," /. Chem. Educ., Vol. 71,
1994,430^32-
Todd P. Silverstein, "Weak vs Strong
Acids and Bases: The Football
Analogy," \ Chem. Educ, Vol. 77,
2000, 849-850.