Chapter 16 Text
Chapter 16 Text
Chapter 16 Text
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<strong>16</strong>.6 Weak Acids 627<br />
Solve: (a) NaOH dissociates in water to give one OH ion per formula unit. Therefore,<br />
the OH~ concentration for the solution in (a) equals the stated concentration of NaOH,<br />
namely 0.028 M.<br />
Method 1:<br />
_ 1-0 X IP"14<br />
[H ] ~ 0.028<br />
Method 2:<br />
= 3.57 x 10~53 M pH - -log{3.57 x lO"13) = 12.45<br />
pOH = -log(0.028) = 1.55 pH - 14.00 - pOH = 12.45<br />
(b) Ca(OH)2 is a strong base that dissociates in water to give two OH~ ions per<br />
formula unit. Thus, the concentration of OH~(aq) for the solution in part (b) is<br />
2 X (0.0011 M) - 0.0022 M.<br />
Method 1:<br />
1 n v<br />
[H+]-<br />
0.0022<br />
= 4.55 X ID"12 M<br />
Method 2:<br />
pOH = -log(0.0022) - 2.66<br />
pH = -log(4.55 x 10~12) = 11.34<br />
pH = 14.00 - pOH = 11.34<br />
PRACTICE EXERCISE<br />
What is the concentration of a solution of (a) KOH for which the pH is 11 .f<br />
(b) Ca(OH)2 for which the pH is 11.68?<br />
Answers: (a) 7.8 X 10~3 M; (b) 2.4 X 10"3 M<br />
Although all the hydroxides of the alkali metals (group 1A) are strong electrolytes,<br />
LiOH, RbOH, and CsOH are not commonly encountered in the laboratory.<br />
The hydroxides of the heavier alkaline earth metals, Ca(OH)2, Sr(OH)2,<br />
and Ba(OH)2 , are also strong electrolytes. They have limited solubilities, however,<br />
so they are used only when high solubility is not critical.<br />
Strongly basic solutions are also created by certain substances that react with<br />
water to form OH~(aq). The most common of these contain the oxide ion. Ionic<br />
metal oxides, especially Na2O and CaO, are often used in industry when a strong<br />
base is needed. Each mole of O2~ reacts with water to form 2 mol of OH~, leaving<br />
virtually no O2~ remaining in the solution:<br />
O2-(aq) + H2O(0 - > 2OH~(aq) J [<strong>16</strong>.22]<br />
Thus, a solution formed by dissolving 0.010 rnol of Na2O(s) inienough water to<br />
form 1.0 L of solution will have [OH~] = 0.020 M and a pH of 12.30.<br />
Ionic hydrides and nitrides also react with H2O to form OH~:<br />
H2O(0 - > H2(g) + OrT^) [<strong>16</strong>.23]<br />
3H2O(/) - > NH3(oj) + 3OH~(aq) [<strong>16</strong>.24]<br />
Because the anions O2~, H~, and N3~ are stronger bases than OH~ (the conjugate<br />
base of H2O), they are able to remove a proton from H2O.<br />
<strong>16</strong>.6 Weak Acids _<br />
Most acidic substances are weak acids and are therefore only partially ionized in<br />
aqueous solution. We can use the equilibrium constant for the ionization reaction<br />
to express the extent to which a weak acid ionizes. If we represent a general weak<br />
acid as HA, we can write the equation for its ionization reaction in either of the<br />
following ways, depending on whether the hydrated proton is represented as<br />
or<br />
or<br />
H2O(/)<br />
HA(aiy)<br />
[<strong>16</strong>.25]<br />
[<strong>16</strong>.26]<br />
Students often confuse a weak add<br />
(small Ka) with a dilute add (low<br />
concentration).<br />
|ohn J. Fortman, "Pictorial Analogies<br />
XI: Concentrations and Acidity of<br />
Solutions," /. Chem. Educ., Vol. 71,<br />
1994,430^32-<br />
Todd P. Silverstein, "Weak vs Strong<br />
Acids and Bases: The Football<br />
Analogy," \ Chem. Educ, Vol. 77,<br />
2000, 849-850.