This page provides problems utilizing the weak acid approximation (i.e. 100 * Ka < Ca, which is very similar to the "5% validity" test). When you press "New Problem", a question will appear to the left of the table. Calculate the [H+] of the solution using the weak acid approximation and press "Check Answer". A message will appear indicating whether your value is correct. If your [H+] is correct, determine whether the approximate value meets the 5% validity criterion. Overall results are kept in the second table. On this page all of the species are weak acids.