Weak Acid and Bases

Weak acids and bases do not ionize very much, so the [H⁺] or [(OH)^-] must be calculated by the equilibrium expression.

This would be easy to know exactly, except that the [HA] is not always equal to the concentration of solute that actually went into the solution. The more accurate way to express the equilibrium expression would be to include in the denominator the idea that the [HA] is really the concentration of solute originally put into solution minus the amount of that solute that ionized. We could represent the amount that ionized by either the [A^-] or the [H⁺], so the denominator should be either [HA] - [A^-] or [HA] - [H⁺]. Let's use the second option for alternative denominator because we want to solve for the hydrogen ion concentration. By the 5% rule, [HA] = [HA] - [H⁺] only if the hydrogen ion concentration is less than five percent of the total solute concentration. This is the case for most weak acids. The only exceptions are when the strongest of the weak acids are in the most dilute solutions.