Acids dissociate by the reaction

. The initial concentration of acid

is

and the acid dissociation constant is

. This is often expressed as

, where logarithms to base 10 are understood. Stronger acids have higher

(smaller

) than weaker acids. The acid constant is defined by

,with

, where

is the fraction of protonation. This implies that

. Thus given

, one can calculate the exact value of

). In this Demonstration, these exact

values are shown as blue curves.

We can see that the approximation for a weak acid is not valid at higher concentrations

. Thus the approximation for strong acids is only applicable if either

or the concentration is small.

This Demonstration is intended for first year chemistry students (in course LC102, University Pierre Marie Curie, Paris, France).