[titration]

Hello hope you can help me because this is quite embarrassing. My younger brother presented me with a problem he have to turn in soon about titration. He have to calculate pKb. I told him to go to the graph (image below) and determine pKa, which would be the titration midpoint (i.e. around 6 right?). It says in the exercise that it is at 25 degrees celsius so pKa+pKb=14 => pKb=8. Is that correct? Hope you can help me

[Borek]

Midpoint is not the end point. Otherwise you are on the right track.

[titration]

I'm not sure I follow, english isn't my first language so you might have to spell it out for me. But you are saying that pKa isn't equal 6 but actually lower?

[titration]

Ohh I think I get it now. Is it the midt point of the "second steep point"? at pH ~4,5? If it is correct would you mind explaining why it is so?

[Borek]

Midpoint is when the substance (acid or base) is titrated exactly in half. Steep part of the titration curve - called inflection point - is where the end point is. So you have to check the final volume from the inflection point, and then find the midpoint at half the volume.

[titration]

So I take the volume at the titration midpoint ~20,5mL. Then I go to 10,25 mL and say that the pKa~9?

[Borek]

Yes.

[titration]

Thank you very much