Question

Hello, I'm wondering what the difference is in method when it comes to finding the pH of a weak acid versus finding the pH of a strong acid? Thanks!

Answer 1

You need to recognize the difference. A strong acid is 100% ionized. HCl is a strong acid. When HCl is placed in water solution this reaction takes place.
HCl + H2O ==> H3O^+ + Cl^-
If you start with 100 molecules of HCl, at the end you will have 100 H3O^+ ions and 100 Cl^- ions and no molecules of un-ionized HCl.

A weak acid is a different story. Weak acids react with water, also, to produce H^+. Acetic acid is an example.
CH3COOH + H2O ==> H3O^+ + CH3COO^-
But the difference is that CH3COOH is NOT 100% ionized. If we start with 100 molecules of CH3COOH, at the end we will have approximately one (1) ion of H3O^+, one (1) ion of CH3COO^-, and 99 molecules of CH3COOH that have not ionized.
Therefore, if we are given 0.1 M HCl, we know immediately that the H^+ is 0.1 M and pH = -log(H^+) = -log(0.1) = 1

But when we start with 0.1 M CH3COOH, we must go through the ionization constant to calculate the pH.
CH3COOH + H3O ==> H3O^+ + CH3COO^-

Ka = (H3O^+))CH3COO^-)/(CH3COOH) = 1.8 x 10^-5 and the
(H^+) = sqrt(Ka*C) = sqrt(1.8 x 10^-5*0.1) = 0.00134 M.
Then pH = -log(H^+) = -log(0.00134) = 2.87.