Kaplan PS section 7 - #21

[akim6890]

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http://s7.postimage.org/gv1vcr3sb/item21.png

There's the question. You also need to know that Phenolphthalein's effect pH range is 8.2-10.

From their answer, isn't it a good indicator?

 

[FormerPhysicist]

The point where [NH3]/[NH4+] is "close to one" should be the half-equivalence point of the titration, which is just inside phenolphthalein's effect range, I think. I wouldn't have known that if I didn't look up phenolphthalein on wikipedia, though.

 

[justhanging]

Just going by what the question says about pheliipn being a poor indicator you can pick D, because it corresponds to the pH buffer region which is the flat part of the curve. Obviously you don't want an indicator that changes colors during that part but rather the steep equivalence part.