# The pH Scale

## You are here

## The pH Scale

The concentration of oxonium ions in solution can be expressed in terms of the pH scale.

**The pH of a solution is the logarithm to base 10 of the reciprocal of the numerical value of the oxonium ion concentration:**

pH = lg 1/H_{3}0^{+}

= -lg [H_{3}0^{+}]

= -lg [H^{+}]

**The pH of a neutral solution can be calculated directly from the ionic product of water.**

Kw = [H_{3}O^{+}] = [OH^{-}] **OR** [H^{+}] = [OH^{-}]

[H^{+}] = 10^{-7}

pH = 7 for neutral solutions.

Acidic solutions have a pH less than 7 at 25^{o}C.

The concentration of hydroxide ions in a solution can be expressed in terms of pOH. This is given by: pOH = -lg[OH^{-}]

It is possible to write an expression relating pH and pOH as pKw = pH + pOH

At 25^{o}C: pH + pOH = 14

Calculate the pH value of 0.001 moldm^{-3} solution of NaOH at 25^{o}C.

[OH^{-}] = 10^{-3}

Therefore: p[OH^{-}] = 3

If pH + 3 = 14, then pH = 11

Calculate the pH of a solution of 0.1M hydrochloric acid at 25^{o}C:

[H^{+}] = 10^{-1} moldm^{-3}

pH = -log10^{-1} = 1