J.R. S. answered • 12/02/23
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HCl is a STRONG acid, and so it will ionize completely:
HCl ==> H+ + Cl-
10-8 M HCl ==> 10-8 M H+ + 10-8 M Cl-
[H+] from HCl = 1x10-8 M
Since the solution of HCl is so very dilute, we must take into consideration the [H+] contributed by the auto-ionization of H2O. Thus, we have...
H2O ==> H+ + OH-
Kw = 1x10-14 @ 25ºC
[H+] = 1x10-7 M = contribution to [H+] by H2O
Total [H+] = 1x10-7 + 1x10-8 = 1.1x10-7 M
pH = -log 1.1x10-7
pH = 6.96
Dejan N.
Yes, indeed. Students would generally fall into two groups when given this problem. The first group would do -log(10-8)=8 and would happily submit pH=8 as the answer. The other group would start the same but would then pause confused. "They taught us pH of acids is below 7, so what did I do wrong here?". Some would just get stuck here not knowing what went wrong, while others would ultimately find the correct answer.12/02/23