pH Calculation

pH CALCULATION

The pH is the negative logarithm of the hydrogen ion concentration

pH = –log₁₀ [H⁺]

Example: Calculate the pH of 0.01 M HNO₃?

Solution:

[H⁺] = 0.01

pH = –log₁₀ [H⁺]

pH = –log₁₀ [0.01]

pH = –log₁₀ [1 × 10⁻²]

pH = –(log₁₀1 – 2 log₁₀10)

pH = 0 + 2 × log₁₀10

pH = 0 + 2 × 1 = 2

pH = 2

pOH: The pOH of an aqueous solution is realted to the pH.

The pOH is the negative logarithm of the hydroxyl ion concentration

pOH = –log₁₀[OH^–]

Example:

The hydroxyl ion concentration of a solution is 1 × 10⁻⁹M. What is the pOH of the solution?

Solution

pOH = –log₁₀ [OH^–]

pOH = –log₁₀ [1 × 10⁻⁹]

pOH = –(log₁₀ 1.0 + log₁₀ 10⁻⁹)

pOH = –(0–9 log₁₀10)

pOH = –(0 – 9)

pOH = 9

Relationship between pH and pOH

The pH and pOH of a water solution at 25^oC are related by the following equation.

pH + pOH = 14

If either the pH or the pOH of a solution is known, the other value can be calculated.

Example: A solution has a pOH of 11.76.

What is the pH of this solution?

pH = 14 – pOH

pH = 14 – 11.76 = 2.24