The 5% Rule

[MJLara]

Applying the 5% rule, explain the conditions under which it is reasonable to make approximations in solving problems related to acid-base equilibria, and provide an example of an acid-base equilibrium where the 5% rule can be effectively applied.

[awang3I]

If K_a or K_b is very small, we can approximate [initial concentration] - x to [intial concentration] in the ICE table. If our calculated x turns out to be 5% or less of the initial concentration, then the approximation is valid.

[Ava Tam 2A]

Hi, there is an example of this rule in his lecture notes. The question states "Given Kb for NH3 is 1.8 x 10^-5, what is the pH of 0.15 M NH4Cl?" You begin by writing out the question, and one can determine that the NH4+ and Cl- ions dissociate (dissolve). One then determines the initial concentrations and compares them to the equilibrium concentrations. X turns out to be less than 5% of the initial concentration, meaning the approximation is valid.