[ahhppull]

Calculate the pH of 5.0E-8M HClO4.  What fraction of the total H+ in this solution is derived from the dissociation of water?

I got an answer of 7.6 pH, but my TA got an answer of 6.92.  He didn't show work, but I think my answer is wrong because its an acid.  How do I do this?  I did the charge balance and set [OH-] = [H+]-[ClO4-].  Then i substituted this into kw =[OH-][H+].

[Borek]

I got an answer of 7.6 pH, but my TA got an answer of 6.92.  He didn't show work, but I think my answer is wrong because its an acid.  How do I do this?  I did the charge balance and set [OH-] = [H+]-[ClO4-].  Then i substituted this into kw =[OH-][H+].

Your approach sounds correct, so I guess there must be some error in your math. Show details of your solution.

Actually it should be 6.89.

[ahhppull]

I got an answer of 7.6 pH, but my TA got an answer of 6.92.  He didn't show work, but I think my answer is wrong because its an acid.  How do I do this?  I did the charge balance and set [OH-] = [H+]-[ClO4-].  Then i substituted this into kw =[OH-][H+].

Your approach sounds correct, so I guess there must be some error in your math. Show details of your solution.

Actually it should be 6.89.

Heres my work

HClO4  H⁺ + ClO4^-
H2O  H⁺ + OH^-
 
Charge Balance
[OH-] + [ClO4-] = [H+]
[OH-] = [H+] - [ClO4-]  (substitute into Kw = [OH-][H+])

Kw = [OH-][H+]
1E-14 = [x-5E-8][ x ]
1E-14 = x² - 5E-8 x
0 = x² - 5E-8x - 1E-14
From the quadratics, I got 2.5E-8 M H+

What did I do wrong?  I did it over and over, but I kept getting the same thing.

[Borek]

0 = x² - 5E-8x - 1E-14
From the quadratics, I got 2.5E-8 M H+

Check your math.

2.5e-8² - 5e-8*x - 1e-14 ≠ 0

[ahhppull]

0 = x² - 5E-8x - 1E-14
From the quadratics, I got 2.5E-8 M H+

Check your math.

2.5e-8² - 5e-8*x - 1e-14 ≠ 0

Thanks, I originally messed up somewhere on the quadratics, but I got it now.