PH Indicator and Endpoint Observations

[i_love_science]

Homework Statement

see below

Relevant Equations

titration curve

A weak base is titrated with a strong acid.

I think that both B and C are correct. The endpoint volume will be lower than the actual equivalence point volume. When the student mistakenly assumes the endpoint volume is the equivalence point volume, the number of moles of strong acid is seen as equal to the number of moles of weak base. The calculated number of moles of strong acid would be lower (because of the lower volume), and the molarity of the weak base would also be lower.

The answer says only B is correct. Could anyone explain why, and where I went wrong? Thanks.

 

[Borek]

I see nothing wrong in your explanation, I would choose B and C as well.

Funny thing is, this titration is still doable, you just have to do it in the opposite direction and stop at the first color change. That's how titrations against phenolphthaleine are done

 

[i_love_science]

Funny thing is, this titration is still doable, you just have to do it in the opposite direction and stop at the first color change. That's how titrations against phenolphthaleine are done

Thanks, though I don't really understand the above part. If you do it in the opposite direction, the color change is still not at the equivalence point...

 

[Borek]

Thanks, though I don't really understand the above part. If you do it in the opposite direction, the color change is still not at the equivalence point...

Which is why you stop at the first sight of color change. Color change is not instantaneous, it takes place gradually over at least 2 pH units. In the case of phenolphthalein it means the color change (when titrating strong acid with a strong base) starts quite closely to the equivalence.

Compare http://www.titrations.info/acid-base-titration-indicators