How can a dilute strong acid and a concentrated weak acid have the same pH

Question

Acids

Answer 1

Strong and weak do not refer to pH. They refer to whether or not the acid completely ionizes (strong), or only partially ionizes (weak). Let's take the example of hydrochloric acid (HCl, a strong acid) and acetic acid (CH3COOH, a weak acid)

CH3COOH only ionizes slightly to ==> H⁺ + CH3COO- (The Ka tells us how much it ionizes. Ka = 1.8x10^-5)

HCl ionizes completely to ==> H⁺ + Cl-

pH = -log [H⁺], so if the [H⁺] is the same from both the HCl and the CH3COOH, the pH will be the same. This can occur if you have a dilute HCl solution and a concentrated CH3COOH solution.

1 M CH3COOH will have the same pH as 4.2x10^-3 M HCl (see below)

1 M CH3COOH: Ka = 1.8x10^-5 = [H⁺][CH3COO-] / [CH3COOH] = (x)(x) / 1 and x = [H⁺] = 4.2x10^-3 M

pH = -log 4.2x10^-3 = 2.37

4.2x10-3 M HCl: [H+] = 4.2x10-3 M

pH = -log 4.2x10^-3 = 2.37

Answer 2

Since the ph of water is 7, dilution can lower the ph of any higher ph acid to the same ph as a lower ph acid.

Answer 3

Because stag and weak refer to the tendency to form H+ ions not the concentration of those ions.