Posted by Clur on Wednesday, May 4, 2011 at 9:14pm.
"Arsenic acid (H3AsO4) is a triprotic acid with Ka1 = 5 103, Ka2 = 8 108, and Ka3 = 6 1010. Calculate [H+], [OH ‾ ], [H3AsO4], [H2AsO4‾ ], [HAsO42 ‾ ], and [AsO43 ‾ ] in a 0.14 M arsenic acid solution."
Okay, so far, using successive approximation, I have determined that [H+] = 2e2M
[OH^‾] = 5e13M
[H3AsO4] .12M
[H2AsO4^‾] = 2e2M
[HAsO4^2‾] =8e8M
But I cannot get the right answer for [AsO4^3]. I thought since x was so negligible that with the amount of sigfigs it would be the same as the Ka3 (6e10) but this is wrong. Can anyone help?

Chemistry  DrBob222, Wednesday, May 4, 2011 at 10:25pm
I think you are not using the number of significant figures allowed. For (H^+) I solved the quadratic and obtained 0.0243 which rounds to 0.024M. That changes OH^ and some of the others, particularly H2AsO4^ as well as H3AsO4. Likewise, I think HAsO4^2 should be reported as 8.1E8.
For k3 = 6.1E10 = (H+)(AsO4^3)/(H2AsO4^2) I would substitute 0.024 for (H^+) and 8.1E8 for H2AsO4^2 and solve for AsO4^3 to at least two s.f. I don't think (H^+) = (HAsO4^2) but you are setting them equal if you let k3 = AsO4^3. Let me know if my reasoning is not correct.

Chemistry  Clur, Thursday, May 5, 2011 at 12:18am
@DrBob222 Thank you so much, you were right!
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