Calculate The Ph and Poh of The Following Solutions

Understanding the pH and pOH of solutions is fundamental in chemistry, biology, and environmental science. This guide explains how to calculate these values and interpret the results.

What is pH?

The pH scale is a measure of how acidic or basic a solution is. It was introduced by Danish chemist Søren Sørensen in 1909 and is defined as the negative logarithm (base 10) of the molar concentration of hydrogen ions (H⁺) in a solution.

pH = -log₁₀[H⁺]

The pH scale ranges from 0 to 14, where:

pH values less than 7 are acidic
pH value of 7 is neutral
pH values greater than 7 are basic (alkaline)

The pH Scale

The pH scale is logarithmic, meaning each whole number change represents a tenfold difference in hydrogen ion concentration. For example:

A solution with pH 3 has 10 times more hydrogen ions than a solution with pH 4
A solution with pH 5 has 100 times more hydrogen ions than a solution with pH 7

Common pH values for everyday items include:

Substance	pH	Classification
Battery acid	1	Very acidic
Lemon juice	2-3	Acidic
Vinegar	2-3	Acidic
Pure water	7	Neutral
Human blood	7.35-7.45	Slightly basic
Baking soda solution	8-9	Basic
Ammonia solution	11-12	Very basic

Calculating pH

To calculate the pH of a solution, you need to know the concentration of hydrogen ions (H⁺) in moles per liter (M). The formula for pH is:

pH = -log₁₀[H⁺]

Where:

[H⁺] = concentration of hydrogen ions in moles per liter (M)

Example: Calculate the pH of a solution with a hydrogen ion concentration of 1 × 10⁻⁵ M.

pH = -log₁₀(1 × 10⁻⁵) = -(-5) = 5

The pH of this solution is 5, which is acidic.

Calculating pOH

The pOH of a solution is a measure of its hydroxide ion (OH⁻) concentration. It is calculated using the formula:

pOH = -log₁₀[OH⁻]

Where:

[OH⁻] = concentration of hydroxide ions in moles per liter (M)

Example: Calculate the pOH of a solution with a hydroxide ion concentration of 2 × 10⁻⁴ M.

pOH = -log₁₀(2 × 10⁻⁴) = -(-3.699) ≈ 3.699

The pOH of this solution is approximately 3.7.

Relationship Between pH and pOH

In water at 25°C, the product of the hydrogen ion concentration and hydroxide ion concentration is always 1 × 10⁻¹⁴ M². This relationship is expressed by the equation:

[H⁺] × [OH⁻] = 1 × 10⁻¹⁴

Taking the negative logarithm (base 10) of both sides gives the relationship between pH and pOH:

pH + pOH = 14

This means that if you know the pH of a solution, you can calculate its pOH, and vice versa.

Example: If a solution has a pH of 3, what is its pOH?

pOH = 14 - pH = 14 - 3 = 11

The pOH of this solution is 11.

Examples

Example 1: Calculating pH from [H⁺]

A solution has a hydrogen ion concentration of 5 × 10⁻⁶ M. What is its pH?

pH = -log₁₀(5 × 10⁻⁶) = -(-5.301) ≈ 5.301

The pH of this solution is approximately 5.3, which is acidic.

Example 2: Calculating pOH from pH

A solution has a pH of 9. What is its pOH?

pOH = 14 - pH = 14 - 9 = 5

The pOH of this solution is 5.

Example 3: Calculating [OH⁻] from pOH

A solution has a pOH of 12. What is its hydroxide ion concentration?

[OH⁻] = 10^(-pOH) = 10^(-12) = 1 × 10⁻¹² M

The hydroxide ion concentration of this solution is 1 × 10⁻¹² M.

FAQ

What is the difference between pH and pOH?

pH measures the concentration of hydrogen ions (H⁺) in a solution, while pOH measures the concentration of hydroxide ions (OH⁻). They are related by the equation pH + pOH = 14 at 25°C.

What is the pH of pure water?

At 25°C, pure water has a pH of 7, which is neutral. This is because the concentration of H⁺ and OH⁻ ions in pure water is equal (1 × 10⁻⁷ M each).

How do I calculate pH from pOH?

You can calculate pH from pOH using the equation pH = 14 - pOH. This works because the sum of pH and pOH is always 14 at 25°C.

What does a pH of 0 mean?

A pH of 0 indicates a very acidic solution with a hydrogen ion concentration of 1 M. Such solutions are extremely rare in nature and are typically found in laboratory settings.

What does a pH of 14 mean?

A pH of 14 indicates a very basic solution with a hydroxide ion concentration of 1 M. Like pH 0 solutions, pH 14 solutions are extremely rare in nature and are typically found in laboratory settings.