pH of Strong Acids and Bases
objectives: define pH, calculate pH, pOH, [H3O+], and [OH-] of strong acids
define “p”:
-log
define pH: -log
of molarity of H+ or H3O+ aka [H+]
Remember from Algebra 2: a log is the exponent that a base needs to be raised to equal a number.
For example—the log of 1000 is 3 because the base 103=1000.
Calculating [H3O+] from pH
If our blue hydrangeas require a pH of 5.5, what is the molar concentration of hydronium?
5.5=-log[H3O+]
10-5.5=3.16x10-6
The pH of swimming pool water should be 7.4 since that is the same as your eyes and other mucus
membranes. What is the molar concentration of hydronium ions?
7.4=-log[H3O+]
10-7.4=3.98x10-8
Calculating pH for Strong Acids
Strong Acids ionize completely. This means that at equilibrium, the molar hydronium ion
concentration will equal the molarity of the acid.
Calculate the pH of a .075M solution of a strong acid
Strong Acids ionize according to the equation
HA  H+
+
A-
.075  .075 + .075
Therefore, simply evaluate –log .075 to determine pH
-log .075 = 1.12
Calculate the pH of a .0005M solution of a strong acid
-log .0005 = 3.30
T. O’Toole
Ionization Product Constant for Water
•
ACID: It should make sense that acids dissolved in water will still have [OH-]—just not nearly
as much as [H3O+]
•
BASE: It should make sense that bases dissolved in water will still have [H3O+]—just not nearly
as much as [OH-]
Calculate the pH of a .010M solution of a strong base
Calculate the pH of a .18M solution of a strong base
Practice
+
-
Calculate the pH, pOH, [H3O ], and [OH ] of a .625M solution of a strong base
[OH-]=.625
pOH=-log .625=.204
pH=14-pOH=13.796
10-13.796=[H3O+]=1.6x10-14
Calculate the [H3O+], [OH-], and pOH of a solution of a strong acid with pH of 3.52
10-3.52=[H3O+]=3.02x10-4
[OH-]=1x10-14/3.02x10-4=3.31x10-11
pOH=14-pH=10.48
T. O’Toole
Calculate the [OH-], [H3O+], and pH of a solution of a strong acid with pOH of 11.66
pH=14-pH=2.34
10-2.34=[H3O+]=4.57x10-3
10-11.66=[OH-]=2.19x10-12
Minute Paper-Organisms in freshwater require a pH of 7.8. Calculate the pOH, [H3O+], and [OH-]
How do I get from where I am to where I’m going?
10 Minute Makeover Customize your learning by choosing from the following options each night:











Complete the appropriate sections on the Portfolio Page
Review the PPT file on the website
QUIA pH practice (http://www.quia.com/rr/335792.html)
QUIA multiple choice practice (http://www.quia.com/rr/345573.html)
Read pages 580-593 in the book
Extra practice problems: page 582 #7 -answer in Appendix C
Extra practice problems: page 586 #9 –answer in Appendix C
Extra practice problems: page 587 #11 –answer in Appendix C
Check out the tutorial sources in the LiveBinder
Complete the SYS Assignment
Review the skills from today’s Train Your Brain
T. O’Toole