First-Year Methods

The method used depends upon the type of substances present after a hypothetical "stoichiometric reaction" in which as much H+ and OH- are used up as possible.

If H+ or OH- is left over, we just ignore everything else.

If only HA, A-, BH+, or B is left over, we solve exactly using "x" in x^2/(C-x)=K.

If a buffer exists (HA/A- or BH+/B), we use the Henderson-Hasselbalch approximation that the equilibrium amounts are approximately the initial amounts and solve for [H+] directly from the equilibrium expression: Ka=[H+](mmol base)o/(mmol acid)o.

These methods are generally not successful when relatively strong "weak acids," such as HSO4– are involved.