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pH of a strong acid

pH is the negative base-10 logarithm of the concentration of oxonium ions. A strong acid, such as hydrochloric acid, is fully protolysed, so the oxonium concentration equals the acid concentration. At 25 degrees pure water is neutral with pH 7. Below 7 the solution is acidic, above 7 basic. The logarithm means that a small change in pH corresponds to a large change in concentration. One unit down is ten times as many oxonium ions.

\mathrm{pH} = -\lg[\mathrm{H}_3\mathrm{O}^+]

[\mathrm{H}_3\mathrm{O}^+] = c

strong monoprotic acid

Symbols

$c$	acid concentration	mol/L
$\lg$	base-10 logarithm

Example

Hydrochloric acid of concentration 0.01 mol/L is a strong acid.

$\mathrm{pH} = -\lg 0.01 = 2$ .

For a strong base it is the hydroxide concentration you know. You find pH using that the product of the two ions is 10⁻¹⁴ at 25 °C.

Practise acids, bases and pH for free →

← Dilution · From pH to concentration →

Part of Chemistry 1: Acids, bases and pH.