Question

Calculate the pH of a 1-L solution containing 0.150 mole of CH3COOH and 0.100 mole of HCl.

Answer 1

Well, we know that acetic acid is a weak acid, so it would be quite odd to add MORE acid--especially strong acid--to a weak-acid solution (generally, you would add base to a weak acid and vice-versa). This strong acid strongly inhibits acetic acid from forming its conjugate base, so there is no ice table to worry about.

In a 1-liter solution, you have 0.150 M acetic acid and 0.100 M HCl solution. The majority of the [H+] comes from the complete dissociation of the hydrochloric acid. You will have:

[H+]total = [H+]HCl + [H+]CH3COOH

However, because acetic acid does not provide much [H+], and its dissociation is actually weakened by the presence of HCl (Le Chatelier's Principle). So the pH is found from the [H+]HCl.

You have 0.100 M HCl, so:

pH = -log[0.100] = 1