AAMC 9, question 17.

[cryhavoc]

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So it says HClO has a ka of 3.2x10^-8. It says this can be used to estimate pka of an acid, and that the pka makes it closest to a pH of 7.

Maybe I forgot everything about acids and bases, but how the heck, just looking at that ka, and I'm supposed to see it has a pH of 7. Would super appreciate anyone walking me through the logic of how I bet to that conclusion.

Thanks!

 

[sazerac]

pH is the negative log of H
pKa is the negative log of Ka
pBanana is the negative log of Banana

Ka of 10^-8 would have a pKa of 8
Ka of 10^-7 would have a pKa of 7
Ka of 3.2x10^-8 is somewhere in between, say 7.5

If you put this substance in a solution of pH 7.5, exactly half of the H+ protons would fall off and float around being acidic, and half would remain stuck to the ClO- ion. If you stuck it in pure water, fewer protons would fall off, because pure water is more acidic than pH 7.5.