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# pOH

## Defines pOH and how it relates to pH.

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Practice pOH
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The pOH Concept

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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  $K_w$ 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.

$\text{pOH} &=14-\text{pH}=14-4.42=9.58 \\\left [ \text{OH}^- \right ] &=10^{-\text{pOH}}=10^{-9.58}=2.6 \times 10^{-10} \ \text{M}$

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.

Relationships between hydrogen ion concentration, hydroxide ion concentration, pH, and pOH.

#### Summary

• pOH is defined.
• Calculations involving pOH are described.

#### Practice

Do all calculations at the link below that involve pOH:

#### 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?