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For example HBr + H2O -> H3O+ + Br-

There is clearly no OH- here, but according to the formula [OH-] = Kc/[H+] indicates that there are OH-. Why is this?

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The formula is correct; note that water contains about $10^{-7}\,\mathrm{M}$ hydroxide, but with the $\ce{HBr}$ added (effectively $\mathrm{pH}=0$) the $\ce{OH-}$ concentration is indeed reduced--now to ~$10^{-14}\,\mathrm{M}$ -- but not to zero. If you had $18$ grams of this solution (with about $6 \cdot 10^{23}$ water molecules), you would still have about $10^9$ hydroxide ions present. Not enough to do a lot of chemistry with, however.

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  • $\begingroup$ Thanks, Zhe. My computer does not allow the browser add-in. $\endgroup$ – iad22agp Dec 16 '16 at 19:24

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