There is a formula linking the values of pH and pOH which allows determining the pH or pOH based on one another.

It is derived from the expression of water ionization, Kw = [H⁺][OH^–].

To get the pH and pOH correlation, we take a negative log of both sides of the equation:

-log [H⁺] + (- log [OH^–]) = – log 10^-14

Notice that the terms on the left represent the pH and pOH of the solution (-log [H⁺] = pH, – log [OH^–] = pOH). Therefore, we can write that:

pH + pOH = 14

Let’s see how this can be used to calculate, for example, the pH from pOH.

For example, determine the pH of a 0.850 M NaOH.

The strategy here is to calculate the pOH and then, using the pH + pOH = 14 relationship, determine the pH.

NaOH is a strong base, so [OH^–] = [NaOH], and therefore, the pOH is:

pOH = -log 0.850 = 0.0706

The pH then is equal to:

pH = 14 – pOH = 14 – 0.0706 = 13.9

As expected, the pH is very high because it is a solution of a strong base.

Check Also

• Definitions of Acids and Bases
• Acid-Base Reactions
• Acid-Base Titrations
• Conjugate Acid and Conjugate Base
• Autoionization of Water and K_w
• The pH and Acidity
• Acid Strength, K_a, and pK_a
• Base Strength, K_b and pK_b
• K_a, pK_a, K_b, and pK_b Relationship
• The pH of a Strong Acid and Base
• The pH of a Weak Acid
• The pH of a Weak Base
• ThepH of Polyprotic Acids
• The acidity of a Salt Solution
• The pH of a Salt Solution
• The pH of Salts With Acidic Cations and Basic Anions
• Acids and Bases Practice Problems