Understanding the Relationship Between pH and pOH in Aqueous Solutions

When dealing with aqueous solutions, the pH and pOH are important parameters that can be used to determine the ion concentration. In this article, we will explore the relationship between pH and pOH, specifically focusing on calculating [OH-] in different scenarios.

Introduction to pH and pOH

The pH of an aqueous solution is a logarithmic measure of the acidity or basicity of the solution. It is defined as the negative logarithm (base 10) of the hydrogen ion concentration, [H ]. The pOH is similarly defined as the negative logarithm (base 10) of the hydroxide ion concentration, [OH-]. The relationship between pH and pOH is given by the equation:

pH pOH 14

This relationship holds at room temperature for solutions of water. It means that if one of these values is known, the other can be easily determined.

Cases in Calculating [OH-]

Case 1: Given pH 3.50

Let's start with an example where the pH of a solution is given:

pH 3.50

To find the pOH, we use the relationship:

pOH 14 - pH pOH 14 - 3.50 pOH 10.50

The concentration of [OH-] can then be calculated using:

[OH-] 10-pOH [OH-] 10-10.50 [OH-] 3.16 x 10-11 M

Case 2: Given pH 4.61

In another scenario, let's consider the pH of another solution:

pH 4.61

The pOH is calculated as:

pOH 14 - 4.61 pOH 9.39

Therefore, the concentration of [OH-] is:

[OH-] 10-9.39 [OH-] 4.07 x 10-10 M

Case 3: Given pH 4.32

In the final case, we have:

pH 4.32

The pOH is then:

pOH 14 - 4.32 pOH 9.68

Using the relationship between pOH and [OH-], we can find:

[OH-] 10-9.68 [OH-] 2.09 x 10-10

Conclusion

Calculating the [OH-] concentration from pH or pOH is a fundamental skill in chemistry, particularly in environmental science, biochemistry, and chemical engineering. Understanding the relationship between pH and pOH allows for the precise determination of ion concentrations, which is essential for maintaining balance in aqueous systems.