8.2 pH and pOH of Strong Acids and Bases

pH and pOH

The acidity and basicity of a substance are measured using a logarithmic scale known as the pH scale. At ${25 \degree C}$, this scale typically ranges from 1-14, although values outside this range are possible.

The acidity and basicity of a substance is directly related to ${[H^+]}$ and {[OH^–]}. Water will self-ionize a very small amount at ${25 \degree C}$:

The equilibrium constant for this reaction, {|Kw| = 1.0 x 10–14}. Therefore, {[OH^–][H⁺] = 1.0 x 10^–14}. To get a number scale of positive integers, the negative log is taken of both sides: {–log([OH^–][H⁺]) = 14}. The logarithms can be separated: {–log[OH^–] –log[H⁺] = 14}.

This defines our pH scale (at ${25 \degree C}$). pH itself is defined as ${-log[H+]}$, and pOH is defined as {–log[OH^–]}. Therefore, pH + pOH = 14.