Question

As the concentration of both hydrochloric acid and acetic acid approach .0001 M, the pH values are converging. Why does this occur?

Answer 1

pH is defined as negative log of hydronium ion concentration. Higher the H⁺ ion concentration lower the pH, lower the H⁺ ion concentration, higher the pH.

When the solution of an acid is concentrated, the hydronium ion concentration is mostly contributed by the acid, but when the solutions of acids become very dilute, the contribution to the H⁺ ion concentration due to auto-ionization of water also becomes significant.

A strong acid HCl dissociates as shown

A weak acid CH₃COOH dissociates as shown

HCl being a strong acid dissociates completely whereas CH₃COOH being a weak acid dissociates partially. At higher concentrations, HCl will have a higher pH than CH₃COOH as former is a strong acid and dissociates completely and latter is weak acid and dissociates partially. Thus for concentrated solutions, H⁺ ion concentration is more for HCl solution than CH₃COOH solution at the same concentration of solutions. At lower concentration like 0.0001 M solution, as auto-ionization of water also becomes significant and hydronium ion concentration due to that becomes a significantly contributing factor to pH, so the pH values of HCl and CH₃COOH become nearly same at lower concentration of their solutions.