The pOH Concept
Ck12 Science
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Printed: May 19, 2015
AUTHOR
Ck12 Science
www.ck12.org
C HAPTER
Chapter 1. The pOH Concept
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The pOH Concept
• Define pOH.
• Perform calculations of pOH.
Can water have healing properties?
Soap Lake has a long history as a healing place. Indian tribes would put aside their rivalries when they came to the
lake to enjoy the high mineral content of the water. In the days before good antibiotics, thousands of visitors would
come and enjoy the soothing waters of the lake. Soap Lake is alkaline, with water quality thought to be similar to
that of the moons of the planet Jupiter.
The pOH Concept
As with the hydrogen-ion concentration, the concentration of the hydroxide ion can be expressed logarithmically by
the pOH. The pOH of a solution is the negative logarithm of the hydroxide-ion concentration.
pOH = -log[OH− ]
The pH of a solution can be related to the pOH. Consider a solution with a pH = 4.0. The [H+ ] of the solution would
be 1.0 × 10−4 M. Dividing Kw by this yields a [OH− ] of 1.0 × 10−10 M. Finally the pOH of the solution equals
-log(1.0 × 10−10 ) = 10. This example illustrates the following relationship.
pH + pOH = 14
The pOH scale is similar to the pH scale in that a pOH of 7 is indicative of a neutral solution. A basic solution has a
pOH of less than 7, while an acidic solution has a pOH of greater than 7. The pOH is convenient to use when finding
the hydroxide ion concentration from a solution with a known pH.
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Sample Problem: Using pOH
Find the hydroxide concentration of a solution with a pH of 4.42.
Step 1: List the known values and plan the problem.
Known
• pH = 4.42
• pH + pOH = 14
Unknown [OH-] = ? M
First, the pOH is calculated, followed by the [OH− ].
Step 2: Solve.
pOH = 14 − pH = 14 − 4.42 = 9.58
−
OH = 10−pOH = 10−9.58 = 2.6 × 10−10 M
Step 3: Think about your result.
The pH is that of an acidic solution, and the resulting hydroxide-ion concentration is less than 1 × 10−7 M. The
answer has two significant figures because the given pH has two decimal places.
The diagram below shows all of the interrelationships between [H+ ], [OH− ], pH, and pOH.
FIGURE 1.1
Relationships between hydrogen ion concentration, hydroxide ion concentration,
pH, and pOH.
Summary
• pOH is defined.
• Calculations involving pOH are described.
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Chapter 1. The pOH Concept
Practice
Do all calculations at the link below that involve pOH:
http://www.sciencegeek.net/APchemistry/APtaters/pHcalculations.htm
Review
Questions
1. What is the formula for calculating pOH?
2. What pOH value is indicative of an acidic solution?
3. A pOH value of 3 would indicate what type of solution?
• pOH: The negative logarithm of the hydroxide-ion concentration.
References
1. User:Jer1128/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:South_shore_of_Soap_Lake_
Looking_North.jpg .
2. CK-12 Foundation - Joy Sheng. .
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