Calculate Poh and Ph for Each of The Following

Understanding the pH and pOH values of aqueous solutions is fundamental in chemistry. These values help determine the acidity or basicity of a solution, which is crucial in various scientific and industrial applications. This guide explains how to calculate pH and pOH, their relationship, and provides practical examples to help you understand these concepts better.

What is pH and pOH?

The pH scale is a measure of the acidity or basicity of an aqueous solution. It ranges from 0 to 14, where values below 7 are acidic, 7 is neutral, and values above 7 are basic. The pH is calculated using the formula:

pH = -log[H⁺]

Similarly, pOH is a measure of the concentration of hydroxide ions (OH^-) in a solution. It is calculated using the formula:

pOH = -log[OH^-]

Both pH and pOH are logarithmic scales, which means that each whole number change represents a tenfold change in concentration.

Relationship Between pH and pOH

In pure water at 25°C, the concentration of hydrogen ions (H⁺) and hydroxide ions (OH^-) is equal, and both pH and pOH are 7. However, in other solutions, the relationship between pH and pOH is given by the formula:

pH + pOH = 14

This relationship is known as the water dissociation constant and is a fundamental concept in chemistry. It allows you to calculate one value if you know the other.

Calculating pH and pOH

To calculate pH and pOH, you need to know the concentration of either hydrogen ions or hydroxide ions. Here are the steps:

Determine the concentration of H⁺ or OH^- in the solution.
Use the appropriate formula to calculate pH or pOH.
If you know one value, use the relationship pH + pOH = 14 to find the other.

For example, if you have a solution with a hydrogen ion concentration of 1 × 10^-5 M, you can calculate the pH as follows:

pH = -log(1 × 10^-5) = 5

Then, using the relationship between pH and pOH, you can find the pOH:

pOH = 14 - pH = 14 - 5 = 9

Examples

Let's look at a few examples to illustrate how to calculate pH and pOH.

Example 1: Pure Water

In pure water at 25°C, the concentration of H⁺ and OH^- is 1 × 10^-7 M. Therefore:

pH = -log(1 × 10^-7) = 7

pOH = -log(1 × 10^-7) = 7

Example 2: Acidic Solution

Consider a solution with a hydrogen ion concentration of 1 × 10^-3 M. The pH and pOH are calculated as follows:

pH = -log(1 × 10^-3) = 3

pOH = 14 - pH = 14 - 3 = 11

Example 3: Basic Solution

For a solution with a hydroxide ion concentration of 1 × 10^-2 M, the pH and pOH are calculated as follows:

pOH = -log(1 × 10^-2) = 2

pH = 14 - pOH = 14 - 2 = 12

FAQ

What is the difference between pH and pOH?: pH measures the concentration of hydrogen ions (H⁺), while pOH measures the concentration of hydroxide ions (OH^-). They are related by the equation pH + pOH = 14.
How do I calculate pH if I know the pOH?: You can calculate pH using the formula pH = 14 - pOH. This is because the sum of pH and pOH is always 14 for aqueous solutions.
What is the pH of a neutral solution?: The pH of a neutral solution is 7. This is because the concentration of H⁺ and OH^- ions is equal in pure water, which is the standard for neutral solutions.
How does the pH scale work?: The pH scale ranges from 0 to 14. Solutions with a pH less than 7 are acidic, while those with a pH greater than 7 are basic. Each whole number change represents a tenfold change in concentration.
Why is pH important in chemistry?: pH is important because it helps determine the acidity or basicity of a solution, which is crucial in various chemical reactions, biological processes, and industrial applications.