# How would a pH curve look like for titration of diluted weak base compared to concentrated one?

Let's say I take $$\pu{80 g}$$ of a weak base, dilute it with $$\pu{50 ml}$$ of water and titrate it with a strong acid. I get a titration curve. Now I take again $$\pu{80 g}$$ of the same weak base but this time I dilute it with $$\pu{200 ml}$$ of water and titrate it with the strong acid. What will be the new titration curve compared to the first one?

And here is what confuses me:

On one hand, the diluted base should have lower $$\mathrm{pH}$$. But on the other hand, at half-equivalence point, $$K_\mathrm{b}=\mathrm{pOH}$$, and since it's the same base, $$K_\mathrm{b}$$ is the same, and thus $$\mathrm{pOH}$$ and $$\mathrm{pH}$$ at half equivalence point should be the same. I am also not sure about the endpoint.

• Welcome to Chemistry.SE. We’d like you to take the Tour to familiarize with the site. – Mathew Mahindaratne Apr 15 '18 at 19:36

The equivalence is, by definition, the moment when $n_{base}=n_{acid}$. As you titrate the same mass of base (consequently the same quantity), your equivalence will be reached at the same point. The only difference, to me, between the two curves will be the fact that you will less see the equivalence when the solution is diluted. First curve is 80g in 50mL, second one is 80g in 200mL.
• your initial pH will change, as it will get closer of the one of water. But keep in mind that the equivalence depends only of $n_{acid}$. Hence, $n_{acid}=n_{base} \Leftrightarrow C_{acid}V_0=C_{base}V_{eq}$. So between your two titrations, pOH at equivalence and half-eq will remain the same, but volumes will change (as the quantity of base to reach the required pOH will change) – Thomas Prévost Apr 17 '18 at 19:44