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September 4, 2015

Homework Help: Chemistry

Posted by Clur on Wednesday, May 4, 2011 at 9:14pm.

"Arsenic acid (H3AsO4) is a triprotic acid with Ka1 = 5 10-3, Ka2 = 8 10-8, and Ka3 = 6 10-10. 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+] = 2e-2M

[OH^‾] = 5e-13M

[H3AsO4] .12M

[H2AsO4^‾] = 2e-2M

[HAsO4^2‾] =8e-8M

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 (6e-10) but this is wrong. Can anyone help?

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