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# pOH

## Defines pOH and how it relates to pH.

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The pOH Concept

Credit: User:Jer1128/Wikimedia Commons
Source: http://commons.wikimedia.org/wiki/File:South_shore_of_Soap_Lake_Looking_North.jpg

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### 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  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.

#### 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.

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.

Credit: CK-12 Foundation - Joy Sheng

Relationships between hydrogen ion concentration, hydroxide ion concentration, pH, and pOH.[Figure2]

### Summary

• pOH is the negative logarithm of the hydroxide-ion concentration.

### Review

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?

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