Question

When diluting a weak acid and a strong acid, why is it that the pH changes more significantly in the strong acid? (consider the equilibrium occurring)
With HCl

Answer 1

With HCl, the acid is 100% ionized and for every factor of dilution (say 2x or 3x or 10x), the (H^+) is reduced by the same amount. That is if we dilute the HCl by 2, the (H^+) is just 1/2 of its original concn; if we dilute HCl by 10, the (H^+) is 1/10 the original concn.
However, in the case of a weak acid, for example CH3COOH (acetic acid), if we dilute the original by 2, more of the acetic acid ionizes
CH3COOH + H2O ==> CH3COO^- + H3O^+ which makes up for some of the dilution of the original H^+.
You can go through the calculations and show that this is so.