Acids and Bases - Calculating pH of a Strong Base

Strong bases have a high pH, but how do you calculate the exact number? KOH is an example of a strong base, which means it dissociates into its ions in aqueous solution. Although the pH of KOH or potassium hydroxide is extremely high (usually ranging from 10 to 13 in typical solutions), the exact value depends on the concentration of this strong base in water. Here is how to perform the pH calculation.

Strong Base pH Question

What is the pH of a 0.05 M solution of Potassium Hydroxide?

Solution

Potassium Hydroxide or KOH, is a strong base and will dissociate completely in water to K⁺ and OH^-. For every mole of KOH, there will be 1 mole of OH^-, so the concentration of OH^- will be the same as the concentration of KOH. Therefore, [OH^-] = 0.05 M.

Since the concentration of OH^- is known, the pOH value is more useful. pOH is calculated by the formula

pOH = - log [OH^-]

Enter the concentration found before

pOH = - log (0.05)
pOH = -(-1.3)
pOH = 1.3

The value for pH is needed and the relationship between pH and pOH is given by

pH + pOH = 14

pH = 14 - pOH
pH = 14 - 1.3
pH = 12.7

Answer

The pH of a 0.05 M solution of Potassium Hydroxide is 12.7.