Ph of the following solutions????

Question

1. 0.001M HCL
2. 0.00001 HCL
3. 1.0x 10^-12 M HCL

Answer 1

You can use the concentration of the HCl because it fully dissociates into H+ and Cl- ions in solution, so the H+ concentration will be equal to the initial concentration, and then use pH=-log [H+] as stated above.
It is also correct that the hydrolysis reaction: H20<->H+ + OH- determines the pH of very dilute acids. Think of it this way- if using the given concentration of an ACID gives you a BASIC (>7) pH then hydrolysis reaction will take over, and the solution will have a pH of 7. The opposite is true for a strong base. If you get a acidic pH (7), then hydrolysis is dominant and the pH will be = to 7.
Note that only strong acids and bases fully dissociate, and if you're looking for the pH of a strong base, the concentration is actually telling you the [OH-] and you need to find the pOH from this. pH = 14 - pOH

Answer 2

pH = -log [H+}

1. pH = 3
2. pH = 5
3. pH = 7 (not 12; the autodissociation of water predominates over the very low concentration of hydrochloric acid).